Use identical sizes in a grid with 5 to 8 cm gaps, unified by one shared thing — frame colour, palette, or subject — and plan the layout on the floor before drilling.
A gallery wall goes wrong in one of two ways: too many sizes, or uneven gaps. Both are avoidable.
Start with a grid. Four or six identical sizes, hung with equal gaps, is the layout that looks deliberate in every room. Use 30×40 cm for a tight grid, 50×70 cm when the wall is over two metres wide.
Keep the gaps at 5 to 8 cm. Wider, and the pieces stop reading as one composition. Narrower, and they crowd.
Use one shared thing. Either the same frame colour, or the same palette across the works, or the same subject. Two of those three is usually one too many.
For an asymmetric layout, anchor it: the largest piece sits slightly left of centre, at eye height, and everything else hangs off it. Line up either the tops or the bottoms — not both, which is impossible, and not neither, which looks accidental.
Plan it on the floor first. Lay everything out, photograph it from above, and rearrange the photo rather than the wall. Then trace each frame on paper and tape the paper up before you drill.
A series is the easy path. Works made as a set share their palette and their handwriting, so they hang together without effort. On this site, the collections pages group work that was made in one run — that's a gallery wall you don't have to compose yourself.