Square Wall Art
Square is the format people reach for last and should probably reach for first. It has no dominant axis, so it does not need height to justify itself the way a tall piece does, or width the way a wide one does. That makes it the answer for the walls that defeat everything else.
The narrow run between two windows. The space above a low sideboard where a tall canvas would tower. A chimney breast, an alcove, the wall beside a staircase where the ceiling line cuts across. In each of these a square reads as deliberate where a rectangle reads as a compromise.
Square also grids better than any other format. Four hung in a block, or three in a row, give you a composition that stays visually clean in a way mixed shapes rarely manage.