I grow a lot of minis in Fl. I use plastic only. As far as adult size, I could give you a better idea if I knew the variety.

Most will stay under 3-4 ft in my area. I have seen pictures of minis that I grow, in an area like California - and they tend to be larger. I treat them like orchids and step up the size of the container only when needed. They will dry out faster in a container.

I use pots that are at least 12 inches high and wide but if I see they are starting to grow out of those I'll go to 16 inches. It really depends on the variety a lot. I have micro minis that have been in hanging baskets for 5 years and haven't outgrown them yet.

Go with plastic or that resin foam. Ceramic pots will crack in the winter in your zone. They're also very heavy when filled with wet dirt and a rose. They can also dry out too fast as well. Make sure the pots have lots of drainage holes and use a good quality light potting soil and they should be beautiful!

Mini roses have a huge root system. I dug one up and the roots were over 4' long!

Most mini's on their own roots will grow very well in 7 1/2 gal nursery pots. Some of the larger bush size mini's require a 10 gal pot.