Start with the room plan: the sofa should leave clear walkways and sit comfortably in proportion to the space. Roughly two-thirds the length of the wall it faces is a useful guide.
Seat depth matters more than most people expect. Shallower seats suit upright conversation; deeper seats suit lounging. Try before you buy, ideally in the clothes you wear at home.
For fabric, consider performance weaves if you have children or pets, and always ask for the rub-count and cleaning advice. Removable covers extend a sofa’s life considerably.