Leave 15 to 25 cm between the top of the sofa and the bottom of the frame, and choose a piece about two thirds of the sofa’s width.
Most art in most homes hangs too high. The habit comes from hanging pieces at eye level in an empty room, then putting furniture underneath.
The rule that works: leave 15 to 25 cm between the top of the sofa and the bottom of the frame. Any more and the picture floats away from the furniture; any less and it looks like it slid down the wall. In a room with low ceilings, stay at the lower end.
A second rule keeps the proportions right: the piece should be about two thirds of the sofa's width. A standard three-seat sofa is around 210 cm, so you're looking for roughly 140 cm of art. That can be one large piece, or two pieces hung with a hand's width between them.
For a single piece above a three-seat sofa, 100×70 cm is the smallest size that holds the wall, and 84×119 cm is the size that makes the room look finished. Above a two-seat sofa, 50×70 cm is usually right.
If you're unsure, the quickest test is physical: cut the size out of newspaper, tape it to the wall, and live with it for a day. The second quickest is to open any piece on this site and use the room preview, which draws the print at its true size against a sofa.
A last thing worth knowing: a picture hung above a sofa is seen mostly by people sitting down, not standing. Sit where your guests sit before you make the final mark.