You will have to wait 30 real days to become an adult. This is actually a good thing, so you can learn how the forest works and how we all interact. Take advantage of it.

I believe you're thinking of the mini-deer spell, not actually a minor deer. Once you become an adult deer when your 30 days is up you can have a fellow deer of the forest cast it upon you to become medium-sized.

Also, please remember to only use art you created yourself.