Start from the largest wall and the two colours already in the room, choose one piece at about two thirds of the sofa width, and let everything else on the wall stay quiet.
The living room is the hardest room to buy art for, because it is the room you look at most and the room with the most competing objects in it.
Start with the wall, not the artwork. Measure the widest uninterrupted wall. Two thirds of that, or of the sofa beneath it, is your target width. Most people buy a third of what the wall needs, then wonder why it looks unfinished.
Take the palette from what is already there. Not matching, echoing. If the room has warm wood and a grey sofa, work with ochre, rust or a soft teal, and the room will hold together without looking themed. Pulling one colour from a rug or a cushion is usually enough.
Choose the mood before the subject. A living room is used for talking, reading, sitting still. Work with a low horizon or an open colour field keeps working in that setting. Busy, high-contrast work photographs well and tires quickly.
One piece or a group? One large piece is easier to live with and easier to hang. A group needs equal gaps, aligned edges, and something shared — a palette, a frame colour, a subject. If you are not sure, buy one large piece.
Check the light before you buy glass. A framed print opposite a window becomes a mirror at certain hours. Canvas has no glass, so it never does that.
Then hang it low. 15 to 25 cm above the sofa. Nearly every room improves the moment the art comes down a hand's width.