if we're talking about reality then i think it would have to be big enough for me to fully cuddle, or even something bigger than me that i can cover myself with. very soft, maybe with a sound box to say cute, silly things and maybe with a baby monitor to record any embarrassing moments.
buuuuut, if we're talking about magical plushies...
again, probably something bigger than me if i want a big caretaker that can overpower me with little effort, or if i'm a very baby mood, something much smaller that can use magic to get the job done, maybe a plush that can change sizes? of course it would be animate, able to walk, talk, etc. having strong hypnosis powers and able to read minds to push buttons most effectively would be perfect, and why not just throw in some reality shifting powers too, one moment you can be perfect potty trained adult, the next your a messy little toddler stuck in a huge crib in their special daycare~
