I've only really experienced driving my 32' class C, so I don't know that I can fully compare it to shorter units. That said, for driving down the road the length really doesn't enter into things much at all, mainly when overtaking another vehicle or merging and having to figure enough space in the lane to fit in. The width and height did take some getting used to. The not very tight minimum turning radius also takes some getting used to; large class C's tend to be especially poor in that particular area. Making a "normal" U turn is usually not practical; going around the block or something similar is nearly essential. (A three or more point turn can also work, but ties up traffic and just generally can be stressful with the needed precise backing up.)
For maneuvering in tight areas, of course a shorter unit is at least a little easier than a longer one, but I haven't had too much difficulty overall. It does take a bit of thinking ahead and of watching things carefully in mirrors and maybe getting out and looking one (or more) times.
In my opinion there isn't much difference in parking between a 25' unit and a 31' unit, though some seem to disagree. In either case you pretty much need two regular parking spaces end to end or several side by side, or else a nice broad area over grass or whatever at the edge of a parking lot to hang the back end over. I haven't found the latter to be an option most of the time, but usually it isn't a great problem in parking lots at stores or malls or whatever (at least in suburbia or rural areas) to find sufficient room unless the place is absolutely packed. There may be some more walking involved than if one were in a car, of course.
In short, my opinion is that unless you have specific limitations (such as the place where you keep your RV is only so big), there's not a lot of reason from a driving or maneuvering point of view to limit oneself to shortish class C's if longer ones would otherwise serve better.
