Start with the room, not the showroom. Measure the wall, the doorways and the stairs, then take off a hand's width: a sofa that fits the plan but not the hallway is the most common return we see.
Sit on it the way you actually sit. A seat depth around 36 inches suits curling up; shallower suits upright reading. Feather-wrapped foam keeps its shape and still feels soft after years, which is why every Harbour cushion is made that way.
Finally, choose a fabric for the life it will have. A tightly woven performance fabric shrugs off coffee and pets, and removable covers mean a spill is a wash, not a replacement.