It depends on your mini roses. I have several and they all behave differently. Like my Rise n' Shine is bushy and about 3 ft tall and 3 ft wide. But the others are about 1 ft. shorter and smaller. Jeanne Kenneally is about 4 ft tall but only 2 ft. wide. Rainbow's End is short, barely two feet tall.
Helpmefind.com is a great tool, type the name of your rose and you'll find infos about it there.

msjam is right in that there is no "average". Each variety is different and even within the same variety there are differences. Her Rainbow's End is 2 ft. Mine sent a shoot up to at least 4 feet. But that was just one eratic cane!
Location can also make a difference. Watch out for HMF size references; they are not always accurate. BUT it will at least give you some idea of where to start. If you want "short", my Michelle Cholet, in a pot, stays under 12 inches and cute as a bug even in our 38 days of 100 degree+ weather.

Yes, you would be better off asking about specific varieties. Because of your zone, some may be smaller, but maybe not.

The roses from the grocery store are "usually" from the Parade series from Poulsen (name of the company that hybridizes these), and can grow anywhere from 1.5 - 3 feet tall.

Thanks folks, these examples of your experiences did help me estimate what to expect.

l & h, you can look up various minis and their sizes on nurseries' Websites. I'll put the Nor' East Miniatures link below, as they have a large selection. (I've also ordered from them, and have been happy with my roses.)

Good luck!

Here is a link that might be useful: Nor East Miniature Roses

From your lips (fingers actually) to the grower's ears! Several of my minis are rescues from the clearance shelf. They are now lovely and thriving, but I doubt they were well marked even when selling for full price.

I have read that some minis aren't even a named variety. I can certainly deal with that, but growers would do well to include some information about expected growth rate.

Oh, I forgot to post the link to Poulsen's site with the Parade roses. They are "named" by Poulsen, but not with official "ARS Exhibition Names" which you will have on roses purchased from other places, and they are usually tagged. Poulsen also has the "Party" and "Patio Hit" series of minis.

Here is a link that might be useful: Poulsen's Parade Series Roses