I can only guess at an appropriate size pot. I observed when I moved a Desiree Parmentier rose this summer that all the roots would easily fit in a 26 or 28 inch pot, and the plant had about five or six canes each ten feet long. So I would guess that you could get a decent sized plant to grow in a pot this size. Buy the best potting soil you can find. I've had reasonably good results (for one season) with Miracle Grow with moisture control.
I have been amazed at how quickly potted plants run out of nutrients. I would buy one of those garden hose feeder attachments http://www.homedepot.com/p/Mir...
and feed the rose a low dose of food with every watering session. Not sure I would use Miracle grow, though.
I'm thinking that when the plant is young something balanced like https://www.amazon.com/dp/B007TFTDPC/
. In the month or so before bloom season in May and in August something closer to https://www.amazon.com/dp/B007TFTACS/
. Make sure the fertilizer has minerals, especially calcium, magnesium, and iron. Or buy some chelated iron, epsom salts, and gypsum to add in tiny quantities when potting and again once a season. Adding a pound or two of clay will probably help with a number of things including evening out moisture level and making feeding less tweeky. I bought some Arm and Hammer kitty litter for this. This is all just a SWAG as I have never grown roses in pots for more than just one season.
You will probably want to water your roses about three times per week through most of the growing season, tapering off a little in mid or late September.
I hope this gets things off to a good start. I look forward to hearing how things turn out.