In terms of motorhome size, for the class C's that typically get rented there's not much of a difference in operating expense for the various sizes—they all get nearly the same (rather abysmal) gas milage, somewhere around 8 mpg (US) or 30 l/100 km at typical highway speeds.
For driving down the road, extra length makes next to no difference in how hard it is. For maneuvering in tight places like gas stations and campsites and parking lots, longer is somewhat more cumbersome and takes a little more forethought. On the whole, though, I wouldn't consider the difficulty in driving to be the main concern for selecting which size to get. Rather, I'd suggest comparing on price and on what provides the interior facilities you need and want.
Do double-check that whatever firm you rent from allows crossing the national border. I would guess that practically any Canadian rental would allow that, but quite possibly some US ones would not.
