One gallon should be sufficeint. They don't need a lot of room. Terracotta dries out too fast in the summertime, if you forget a regular watering time.

Here is a link that might be useful: Propagating Perennials

I agree with the one gallon sized pots. You generally don't want to use too large of a pot since the more soil in the pot, the more water the soil can hold and you may have issues with rot. If over the course of the season you begin to see roots coming out of the bottom of one gallon pots, you can then move them up to larger sized pots. I agree that terracotta will dry out way too fast. I

f you have any old 1 gallon sized pots from perennials you have purchased, they will suffice. Since you live in zone 9, digging a hole in the ground and sinking the pots in the holes will help prevent the pots from drying out as fast as keeping them sitting on top of the soil.