In reference to expanding this Room's Dimensions; you could theoretically make a series of 30×30×30 Demiplanes lined up next to each other in any direction, then just rip their barriers apart to connect them; a powerful enough caster could likely perform this without too much effort. Phlogiston is what technically makes the creature/object ethereal and provides omnidirectional movement (2 separate effects), if you are paying attention you will recognize the reaction to “willpower” (Intelligence, Mental Fortitude, Mental Prowess). Strong Intelligence when visiting the Deep Ethereal using Demiplane Spell will give you a stronger/faster character amidst the Phlogiston.
- Possible to connect these rooms? Yes.
- Easy to connect these rooms? Nope.
Supporting and related references to source material can be found below.
I am going to pull from my gathered information listed in this Stack Exchange question over here to answer your original question very quickly yet extremely thoroughly, though first I will preface a few things:
- I will preface stating I presume you are using the Great Wheel Cosmology
- Mordenkainen's Magnificent Mansion doesn't expand space, this would actually get Conjured, unlike the Demiplane, you wouldn't die from this, just a regular portal to a temporary pocket dimension.
- in the Deep Ethereal (movement, breathability, travel)
- from “Phlogiston” (D&D equivalent of “Space Ether”, AKA Protomatter)
- No, you can't merge the Doors (you would require multiple mages to cast 4× anyway)
- You forgot Squeezing, you should actually update your question to reference “No Elephants” as opposed to “No Ogres” because Ogres (& donkeys) could squeeze their way past this door.
Now, onto my main bulk reference citation to help thoroughly answer your original question (I looked at this question's Edit History), again; presuming you are using the Great Wheel Cosmology:
Page 48 in the DMG reads:
The Ethereal Plane is a misty, fog-bound dimension. Its "shores," called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. Visibility in the Border Ethereal is limited to 60 feet. The plane's depths comprise a region of swirling mist and fog called the Deep Ethereal, where visibility is limited to 30 feet.
Characters can use the etherealness spell to enter the Border Ethereal. The plane shift spell allows transport to the Border Ethereal or the Deep Ethereal, but unless the intended destination is a specific location or a teleportation circle, the point of arrival could be anywhere on the plane.
From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy. Ethereal denizens watch the plane as though peering through distorted and frosted glass, and can't see anything beyond 30 feet into the other plane. Conversely, the Ethereal Plane is usually invisible to those on the overlapped planes, except with the aid of magic.
Normally, creatures in the Border Ethereal can't attack creatures on the overlapped plane, and vice versa. A traveler on the Ethereal Plane is invisible and utterly silent to someone on the overlapped plane, and solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal. The exceptions are certain magical effects (including anything made of magical force) and living beings. This makes the Ethereal Plane ideal for reconnaissance, spying on opponents, and moving around without being detected. The Ethereal Plane also disobeys the laws of gravity; a creature there can move up and down as easily as walking.
To reach the Deep Ethereal, one needs a plane shift spell or arrive by means of a gate spell or magical portal.
Visitors to the Deep Ethereal are engulfed by roiling mist. Scattered throughout the plane are curtains of vaporous color, and passing through a curtain leads a traveler to a region of the Border Ethereal connected to a specific Inner Plane, the Material Plane, the Feywild, or the Shadowfell. The color of the curtain indicates the plane whose Border Ethereal the curtain conceals; see the Ethereal Curtains table.
Traveling through the Deep Ethereal to journey from one plane to another is unlike physical travel. Distance is meaningless, so although travelers feel as if they can move by a simple act of will, it's impossible to measure speed and hard to track the passage of time. A trip between planes through the Deep Ethereal takes 1d10 × 10 hours, regardless of the origin and destination. In combat, however, creatures are considered to move at their normal speeds.
From this Data we found:
The Ethereal Plane also disobeys the laws of gravity; a creature there can move up and down as easily as walking.
Additionally, this extra information prevents certain results: Exertion to attempt to destroy Walls gets limited from:
Distance is meaningless, so although travelers feel as if they can move by a simple act of will, it's impossible to measure speed and hard to track the passage of time.
Depending on if you follow specifically Forgotten Realms Lore, you would find this infodump on the Ethereal Plane useful:
Cosmology: Great Wheel
In the Great Wheel cosmology model, the Ethereal plane existed adjacent to the Prime Material Plane and connected it to the Inner planes (the Elemental planes plus the Energy planes). The Ethereal touched the Prime at all points that were located within crystal spheres through what was called the Border Ethereal. It was unknown whether the Border Ethereal touched the Prime in the phlogiston Citation: 6 Citation: 13.
The non-Border region was called the Deep Ethereal. While in the Border Ethereal, a traveler could still see into the adjacent plane but only dimly and not very far, whereas those on the bordering plane could not see the traveler without magical detection. Verbal communication was not possible between the Border Ethereal and the bordered plane. The Ethereal was unique among the many planes in that an individual could exist in two planes simultaneously: the Border Ethereal and the adjacent plane Citation: 6.
When travelers crossed into the Border Ethereal, they and all their possessions were converted to their Ethereal equivalents—metal became ethereal metal, flesh became ethereal flesh, and so on Citation: 14 —allowing free movement (in most cases) in any direction through the solid matter of the adjacent plane Citation: 6. Since everything was permeated with ethereality, an air-breathing creature could breathe ethereal air and could not drown in an ethereal lake nor be crushed by an ethereal rock. However, not all in the adjacent plane was insubstantial. Living things larger than one-celled animals generated an aura that radiated around them and prevented passage to their interior, so an ethereal traveler could not place a weapon inside a living creature where it would materialize and cause damage. A jungle would be an extremely torturous maze to navigate; it would be much easier to float above the vegetation or pass below its roots. Dense metals such as lead or gold also prevented passage of ethereal matter. And finally, some magic spells and alchemical mixtures could form an effective barrier Citation: 14.
Travel in the Ethereal and Border Ethereal was accomplished by force of will—you wished to go somewhere and you did, at your normal rate of movement. There was a sense of up and down but no real gravity existed. Objects released from possession would hover where they were dropped; it was impossible to fall in the Ethereal plane Citation: 15. To get to another plane, one had to pass through a curtain of vaporous color into the Deep Ethereal, then traverse that region until reaching the curtain that demarcated the Border Ethereal of the destination plane. Each Inner plane and demiplane had a curtain with a unique color. The Prime Material plane's curtain was turquoise Citation: 14. If the (usually magical) ethereal effect wore off while a traveler was in the Deep Ethereal, he would immediately be forced through a random curtain at a random location in the Border Ethereal and deposited on the plane which it bordered Citation: 15.
When you passed through a curtain into the Deep Ethereal, time slowed down to one tenth the rate it flowed in the Border Ethereal and the plane that it bordered. For every ten hours spent in the Deep Ethereal only one hour passed on the other side of the curtain. Metabolic and other natural processes slowed down also, so it did not feel like ten hours, when you crossed back through a curtain into a Border Ethereal you were only an hour older and an hour hungrier Citation: 14.
The Deep Ethereal swirled with large blobs of protomatter (imagine a cosmic lava lamp) which could form a demiplane when a critical size was reached. Powerful wizards, technologists, or demigods could also bend the proto-matter to their will and create a demiplane Citation: 16. These nascent planes might exhibit some of the characteristics of the Inner planes or the Prime, but with their own rules of gravity, material make-up, etc., and even support life. Most demiplanes eventually collapse into themselves and break up or merge with another Inner or Prime Material plane Citation: 17.
Citation: 24 in case you want to Review my source referencing these citations.