It depends on your experience with motorcycles let alone never riding a dirt bike;
Ridden motorcycle and good:
You could go for anything between the 250 and 650 range. You could probably have a bit more fun with a two stroke because they have A LOT more power.
Never ridden motorcycle:
Get a 4 stroke 250. It will be fun to play around on, too. It just won't accelerate as quickly or go as fast, but it'll work!