Roots will normally grow a little past the weep line. They will bloom in pots but RAPIDLY outgrow the pot. In a pot, during hot summer they will need LOTS and I do mean lots of water.

you would need at LEAST a 30 gallon pot& the roots would still grow out into the ground.They send up new shoots each year so thats why some are wide unless older ones get pruned out.

I bought my confederate rose in a 5 gallon container. it was over 6 feet tall and blooming at the nursery. I planted it out, but that at least shows that they can flower in a pot. they are vigorous though, and it would probably take a lot of work to keep one in a container several years. but they root easily too, so you could always have one coming along.