I’m three years in with my attempt at an espalier micro-orchard across my back fence with 7 trees, and had the same exact questions about roots when I was looking to get started. My top wire is 2.3 meters up (somewhat higher than your target height), with about 2.2 meters horizontal space allocated per tree. The forms I am shooting for are an alternating pattern of horizontal cordon and chevron with central leader.
You will learn a lot the first couple years, and probably wish you had done some things differently! At least that is my experience 
My location is an urban lot in Somerville, which is a neighboring town to Boston. So less cold in winter than the folks in Maine and even a bit warmer than @mamuang who is further inland, but still New England.
I think trying to match vigor of the rootstock/scion combo to the space available is a good idea. With a tree that wants to grow way bigger than the space, you are going to have to do a ton of pruning and even then it may take a long time to settle into fruiting if it ever does. You will continually be fighting the natural tendencies of the tree, and let’s say you neglect the trees for a year or two or move away. It would be nice if it became a disaster slowly rather than quickly, right? Of course it is tough to match the tree to the space ahead of time given all the factors at play, and undershooting on vigor is also not great. My current approach is to shoot for something likely to have a bit more vigor than needed.
In my tiny orchard even, the trees on the west side of the fence are doing about as expected vigor wise, but the ones on the east side are not. The east side gets more afternoon sun so it is drier and hotter (haven’t hooked up irrigation yet). So far all the scions I have used are fine for espalier, though I am walking through to pinch unwanted growth and tie shoots to the trellis every couple days through the growing season. Some varieties seem to take less wrangling than others. For instance Goldrush is very well behaved and easily does what I want. Sweet Sixteen is growing really thick shoots and twigs, which makes me have to shape new growth sooner than I might like, and makes it maybe a little harder to graft other varieties on. Ashmeads Kernel is growing very slowly; I probably undersized the rootstock.
I have a number of M.111/G.11 interstems, which Cummins was offering at the time I started. In retrospect I think I’d opt for a single mid to small size root. I have a couple G.30 and one G.222, and next spring I’m going to add another Goldrush on G.222 or a Redfield on G.935 to replace an Opalescent on interstem that got completely destroyed by fireblight this summer.
It is still too early to say if I picked the right size roots. But in general for your application I’d say something like G.30, G.202, G.222, or G.935 for average or lower vigor scion, maybe B.9 for high vigor. But take my advice with a spoonful of salt because mostly I am a novice and don’t have much experience. The other limitation is of course what you can find, unless you want to graft your own (which is more do-able than most people would think, but it will add an extra year or so).
Here are some writeups on my blog about my espalier project:
I have not gotten to my year 3 writeup yet, but if I remember I’ll edit this post to include it when I post it.