Most people measure the wall and forget the walk. They find a sofa that looks right against that stretch of living room, pay for it, and then discover it cannot get past the stair turn or through the apartment door without the delivery guys taking the legs off and shrugging at each other. Measure the doorways, the hallway width, the elevator if there is one, before you measure anything else.
Where a sofa goes depends on how you actually use the room, not how it looks empty. A deep, low sofa suits a den where people pile on for a movie and nobody sits up straight anyway. A tighter three seater with real arms works better in a smaller living room you want to use for actual conversation, or in a rental where you will move it again in two years and don't want to fight a corner piece down a narrow hallway. Corner and large sofas earn their size in open plan rooms and family houses where the sofa is doing double duty as the place everyone ends up on a Sunday afternoon, but in a smaller space the same piece just eats the room alive. For sizing, leave at least thirty inches of walking space in front of it, measure the exact spot with tape on the floor before you buy, and go a few inches smaller than you think you want rather than bigger. A sofa that looks modest in a showroom photo often looks enormous once it is backed against your actual wall. What separates a sofa that lasts from one that sags by year two usually isn't the fabric, it's underneath it. Pick it up at one end if you can, or ask about the frame. Hardwood or plywood frames with screwed and glued joints hold their shape, particleboard and staples do not. Serpentine springs are fine for lighter use, pocket springs or webbing handle daily life better, and cushions filled with a foam and fiber mix keep their plump longer than foam alone, which goes flat and stays flat. Run a hand along any seam and check it's double stitched, not just glued or stapled at the back where you won't see it until it's already coming apart.
A good sofa earns its softened, broken in look over a few years, cushions settling into the shape of whoever sits there most, which is a feature, not a flaw, as long as the frame underneath isn't the thing giving up first. Fabric covers that unzip and wash save you more grief than any stain guard spray ever will, and a weekly once over with the vacuum attachment keeps crumbs from working down into the seams where they do real damage. If I were starting from nothing, I'd go with a mid gray in a hardwearing weave, forgiving of spills, dog hair and bad lighting, and honest about the fact that nobody's living room stays pristine for long.