How does Disguise Self manifest for creatures of very large, very small, or unusually-shaped bodies?

Question

Some magic items, like False Face allow you to change form into a humanoid creature with a specific size, but the only requirement is that the creature has an appropriate slot for the magic item.

The spell description for Disguise Self says it does not change tactile features, so it's safe to assume that a small creature turning into a medium creature cannot be felt if you run a finger from its head to toes. (If you could feel it, that would either mean the size of the creature has changed, or the tactile features of the creature have changed.)

If I made an animal wear it, how would the body move and interact or collide with the environment, and what would happen if I made a colossal creature wear it? Would it turn into a medium humanoid floating a hundred feet above ground, surrounded by an invisible force field that feels like flesh while leaving giant footprints?

If I made a flying skull wear it, and got it to land on the floor, would the body fall right through the ground, leaving only the face exposed?

This question sounds a bit ridiculous — but even basic items like a hat of human guise let you change into small humanoids, but specifically says it can be worn by medium humanoids — does the small humanoid body levitate off the ground? How do acrobatics checks to fit in small spaces work if the small humanoid is normal? Why don't they get a circumstance bonus to AC from having an odd shape? In fact, the disguise self spell alone lets you grow an inch shorter or taller, so what would happen if you placed an apple on your head and got an archer to shoot it? Would the apple fall directly into your face or hover above your head?

Answer 1

Alright, let's start saying that your assumption to apply real world physics to explain how unexplained spell effects work isn't going to work very well, specially in D&D/Pathfinder.

Now, Pathfinder has an two unwritten rules that say: "unless something says you can do it, you can't." and "specific rules override the ruling of the generic rule; and when lacking a rule, use the general ruling". As I said, those are unwritten (yet commonly accepted) but will help you a lot when reading things as written, and I need to mention them to further explain my case bellow.

Now, let's tackle those illusion spells that cause so much confusion at every table. The spell description usually says what can be seen, anything else you should check the rules for Illusion in the Magic chapter, and if it can't apply, the general rules for spells (you would be surprised how many people actually incorrectly use certain spells by ignoring that).

Illusions says:

Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, not see things that are there, hear phantom noises, or remember things that never happened.

Disguise Self is a glamer effect:

Glamer: A glamer spell changes a subject's sensory qualities, making it look, feel, taste, smell, or sound like something else, or even seem to disappear.

That said, we can see that even being an illusion, Disguise Self would actually make you feel like the illusion is real if you fail your saving throw. But in the case of Disguise Self, the part about tactile (touch) and audible (sound) perception isn't changed like normal glamers.

As for disbelieving it, Saving Throws and Illusions (Disbelief):

Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion. A successful saving throw against an illusion reveals it to be false, but a figment or phantasm remains as a translucent outline.

Notice how glamer isn't listed here? So you don't get a translucent outline at all, it simply disappears to your character.

So, even if you can touch and feel the target if you reach out, the illusion works just as written, it doesn't clip into buildings, it doesn't fall into the ground, it doesn't levitate because the item is worn by a smaller creature pretending to be a large one. How would an ogre feel when disguised as a halfling? If you tried to touch him you would feel something on the way (the ogre's body) and would most likely get a saving throw to notice the illusion and see the real ogre. But on a failed save, the GM could say you feel a force field protecting him (or something along those lines).

As for a dog wearing a hat of (animal) disguise, it would look like a normal person walking around, even dodging obstacles so the illusion can remain convincing. You would only get a saving throw if you tried to touch that person, or if the dog barked (a human barking? what is going on here...).

Why? Because the spell description does not say so. It isn't intended to do so; if that was the case they would have spelled it out to work that way. The spell doesn't work like a hologram that has a fixed center of emanation and must be correctly placed or otherwise it would "look weird", because the spell is altering the senses of those who see it. For contrast, I believe the technology guide does have an illusion spell that works exactly like a hologram, by the way.

Happy gaming. :)