Many things rely on size or would need to be addressed
The heavy property:
Small creatures have disadvantage on attack rolls with heavy weapons. A heavy weapon's size and bulk make it too large for a Small creature to use effectively.
It does not actually have any effect on Tiny creatures, only Small ones. This just needs to be ...
If your starting point for this idea was based on the assumption that because reduce can turn a Small creature Tiny and therefore Tiny creatures must be balanced, then your starting point is flawed.
The enlarge/reduce spell is a limited time and cast via a limited resource. Saying that the short-term, limited use is balanced as an always-on option is simply ...
Don't confuse the Grappled condition with the "grappling" attack option.
When you want to grab a creature and hold on to them, the "grappling" attack option is how you accomplish that. The rules for doing this include a size limit on the target. If you succeed, you apply the grappled condition to the target.
But the size limit is a function of the ...
They Work as usual
The rules don't specify if the grapple is independent of size, but as the grappling rules for monsters say:
Many monsters have special attacks that allow them to quickly grapple prey. When a monster hits with such an attack, it doesn't need to make an additional ability check to determine whether the grapple succeeds, unless the attack ...
Yes, unless the description says otherwise
The grapple ability of many monsters do not necessarily work the same way as a PC grappling. Although the resulting grappled condition is identical, the process by which the creature gets grappled just follows the description as given. There is no general rule that says otherwise.
Note that the Giant Frog's "...
The Huge or Larger creature can move through the sphere
The spell states:
On a successful save, a creature is ejected from that space to the nearest unoccupied space of the creature’s choice outside the sphere. A Huge or larger creature succeeds on the saving throw automatically.
So the creature can choose to go back to the square it was just in, or to ...
A Merrow's Harpoon damage does not vary with player size
The linked answer points out that the rules for players and for monsters are different and, thus, that there is no way of knowing what damage a harpoon does when a player wields it-- it has no stats for that. The GM could use the improvised weapon rules, but that's not really appropriate because it's ...
Powerful Build does not affect weapon damage
Powerful Build is a straightforward feature:
You count as one size larger when determining your carrying capacity and the weight you can push, drag, or lift.
The only effect of Powerful Build is to increase your carrying capacity (along with several related numbers that all depend on your carrying capacity). ...
The rules say nothing about this
Nothing in the rules mandates anything about:
what happens if a rope you are caught in is pulled really hard
what happens if you shrink or grow while grappled or restrained
what happens if you lose a limb or your head or whatever
what happens if you want to tie someone to something with a rope
In fact, the rules hardly ...
The spell doesn't reduce them further.
You aren't missing anything, there are no rules for what to do in this situation. The spell could have been better worded.
However, I think it's logical to assume that if Enlarge cannot make them bigger if something stops it, then Reduce can't make them smaller if something stops it.
If you tied a creature’s limbs with rope and the rope was sufficiently tight to hold the limb, when the limb shrinks it will slip out of the binding (since it shrinks in all dimensions). The rope was originally tied around a girth double the size of the reduced limb; the new girth of the reduced limb could easily slip out.
The reduce spell would ...
Wild Shape doesn't care about your size.
Lini, the Iconic Druid, is a small-sized character. There isn't any annotation that pointed out that she deviated from the normal rules of Wild Shape and, as written, she is more than able to polymorph into medium-sized animals without any issue.
If such important limitation was intended, I believe it would be ...
As the other answers make clear, "Tiny" in fact covers a very large range of sizes, since it's the smallest one stated and yet must include everything from a cat to a spider.
It's weird that the Monster Manual doesn't give more info on the fantasy creatures. We already know how big a cat is, and that spiders vary widely, but calling an Imp just "tiny" ...
It's up to your DM
The rules for Hiding state:
The DM decides when circumstances are appropriate for hiding.
The only constraint is:
You can't hide from a creature that can see you clearly
One person usually can't hide behind someone else. However perhaps a skinny person could hide behind a fat person. Perhaps a person could hide in a crowd of ...