The easiest way to choose a canvas size above a sofa is to test the dimensions on the wall before buying. A size that sounds large in a product list can look quite small above a wide piece of furniture.
Measure the sofa, the clear wall area and any nearby lamps or shelves. The useful space is the area the artwork will occupy comfortably, not necessarily the whole wall from corner to corner.
Start with a paper template
Cut paper to the proposed canvas dimensions or arrange smaller sheets into the same rectangle. Attach it using a wall-safe method suitable for your finish. Stand back from several places in the room.
Ask whether the picture looks connected to the sofa, whether there is breathing room around it and whether the height is comfortable when seated. There is no universal percentage that makes every room correct. A low ceiling, shelving or an off-centre sofa can change the balance.
Measure groups as one arrangement
Three prints need to be measured together, including the gaps. For example, three prints each 30 cm wide with two 5 cm gaps occupy 100 cm overall. Those numbers describe an arrangement, not a required spacing rule.
Try the whole group in paper. Keep the gaps deliberate, and check that the outer edges have enough room. The combined composition matters more than whether each print seems large by itself.
Let the photograph influence the size
A wide landscape may suit one horizontal canvas. A portrait may be better as a taller piece beside another image. Avoid choosing dimensions first and then discovering that the crop removes the most important part of the photograph.
Check the source file too. Enlarging a small, heavily cropped image can make its softness more visible. If a very large single canvas is unsuitable, a smaller print with more surrounding wall space can be a better result.
Compare canvas sizes and prices, then return to your paper template before choosing. Once the scale feels right in the room, the purchase becomes a much simpler decision.