I've become a big fan of Dwarf Giants (Enano Gigante). It's a variety of Musa acuminata and readily available. This is my first year growing it and it has not fruited but I understand its fruit is creamier and sweeter than the store-bought. Not sure of size but assume the fruit is medium-sized or larger. What I like about the banana is just how robust and fast growing this variety is. It also suckers heavily! I put mine into the ground recently as it quickly outgrew its container. I also like the super dwarf cavendish but I don't think this will produce the size fruit you want. The super dwarf may be dwarf but it is also a quick grower and freely suckers. Be prepared with large containers--even for the 'dwarfs' as they grow quickly.

With respect to your question on heating, it really depends on which variety of bananas you're growing, whether the bananas are green and growing OR dormant. With kinds of temperatures are we talking about? My zone is also 7 and my garage is detached and minimally heated by a tiny space heat--just to keep it above freezing. The only bananas that will survive this green are basjoos and Chinese Lasiocarpa (not grown for bananas). I can't overwinter M. acuminata varieties in my garage without serious die back or rotting of the tuber--so these have to come into the house and I let them go dormant and put them into the basement. Good luck with the bananas--they are fun!

Here's a picture of the Dwarf Giant.

Here is a link that might be useful:

Here's a picture of the Dwarf Giant banana.

Here is a link that might be useful:

I live in Carthage and have had good results with Dwarf Orinoco, I also grow Ice Cream, Raja Puri, Green Red, Praying Hands and several ornamental bananas. I have several Dwarf Orinoco pups if you wish to try some as well as some of the other varieties mentioned.