I would start small and step up because most prairie plants don't want to stay wet too long. I don't think you will need to go over a one gallon size before planting out. You can make a pot that is supposed to prevent root circling by taking a pot to a band saw and cutting a slit up the side about 2/3 of the distance from tine large drain hole in the side bottom to the rim. I have done that with cottage cheese cartons which are about 1 qt. size. I use a heated screwdriver to make the main drain holes around the edge of the pot and then take to the band saw to make the side cuts. You would need to use the more rigid pots.

Best to your research on how well they transplant beforehand.

PS... One article I read mentioned cutting the bottoms (and tops) out of 1/2 gallon milk containers...putting newspaper over the bottom while filling and planting and using these for root pruning nut trees. I used rubber bands to hold the paper on until it decomposes. I have some dogwood, Japanese Maple, and Pin Oak started. They must be stood on hardware cloth, of course. I have planted trees grown this way, and they certainly have a huge root system but no circling.