If all you get is one picture per wall, then it should definitely be quite large - e.g. something rectangular at least 3/4 as wide as the couch and somewhat less high than the width. And something square that takes up quite a lot of the dining room wall - big! But why not have a whole lot of things, or at least a good handful in each space? None would have to 'match' necessarily (especially if they're art vs deco junk) but the frames could match, or else at least one common color or color family in each. People always tend to hang stuff that's too small, and you could really almost fill the walls (except for where pix are separated by a few inches) with art and it would look great. Don't be too 'polite' about it - you'll be surprised at how terrific it'll look in the end!