With the wall, not the picture. Measure the usable width and height first, then decide whether the space needs width or height. Once the shape is settled, subject and colour are quick decisions - the other way round tends to end in a piece that does not fit.

Above furniture, aim to fill about two-thirds of the width of the sofa, bed or sideboard beneath it, leaving 15-20cm of clear wall between the two. On an empty wall, measure the full usable span and take at least 15cm off each end.

Because most homes have more long, narrow wall than square wall, and almost no one makes art for it. A hallway, a staircase, the strip above a sofa and the gap beside a door all share the same problem: a standard frame either floats in the middle of the space or does not fit at all.

Centre the artwork at roughly eye level, around 145-150cm from the floor. Over furniture, measure from the furniture instead and let the 15-20cm gap set the height. On a staircase, follow the line of the stairs rather than the ceiling.