For a long time, I've thought the Minecraft bee was too big, while the baby bee is a perfect size. Is there a way for me to disable mob growth, mob aging, or set a mob's size? I've looked, and I cant find anything on the wiki. If not, I think this would be a cool addition to the spawning rules because we could use it to naturally spawn giant zombies with working animations or size down mobs.

You may look into the NBT of the mob.
Maybe you can change the value to a high negative one.
So it will never reach the time needed to grow to an adult.