when will it move out?

Question
how big does a baby corn snake have to be before it can move out of the smaller cage into a bigger one

Answer
There isn't really a length limit, more a cage limit compared to him.

You don't want the next cage to be too much bigger, because that can cause some stress on the snake. 10gallon to a 30 gallon may be too much of a jump. But a 10g to a 20g would be ok. If it is a jump like 10 to 30, then make sure he will have plenty of hiding places in the new tank. This will limit the stress.