You're definitely doing the right thing by considering size before buying. Too often we hear from people who can't figure out how to arrange their furniture because it's really too big for the room. I suggest you sketch your room, marking dimensions and doors and windows. Then take it to West Elm, Pottery Barn, and Crate & Barrel, along with a local store or two, to see what the helpful staff at these stores suggest. Or you could post your sketch here and we'd be happy to make suggestions.

Along with a sofa, I would purchase two chairs, which makes for a wonderful combination that you can arrange in several ways. DO NOT, oh please, do not, buy a loveseat! A sofa and loveseat combo can only cause problems in arranging them.
Carolyn Albert-Kincl, ASID