Some paeonies are semi shrubby (tree paeonies) but yours doesn't look like one of them. I'd get it in a larger pot now before it puts on much more growth.

If you can give your location people can make suggestions for other perennials you could grow.

Depending on your zone growing them in pots is doable! You'll want a bigger pot, though. If you're in a warmer zone you can just lay the pot on it's side and leave it outside for the winter, if you're in a cold zone the constant freeze/thaw/repeat cycle is deadly to them so you'll want to put it in a storage room, garage, etc where it's cold but not freezing in the winter. The main cause of failure with pots in the summer is that they get really hot when left out in full sun (peony's preferred location!) so shade the pot if you can, double potting it and using mulch should work to keep the roots cool. Don't water until the soil dries out to at least an inch or so down, overwatering is super easy to do in pots as well. Good luck!!!

I am a zone 7B. I also found out it is a Margaret Truman peony. What size pot would you recommend?

Depends on how long you plan to leave it in the same pot. If the root was a small one, like the bagged ones from the big box stores, you can leave it like it is for this year. MT is a large lactiflora and they typically have very large roots, can go out to a 2' diameter easily and down a good 18"-24" when mature. Think this one gets up to 30" or so tall.

hosta grow well in pots too