Start with an anchor: your largest print (50×70 or bigger), hung at eye level. Build outward with 2–3 30×40s and a few A4s, keeping gaps consistent — 5–8 cm reads as deliberate.
Unity comes from one repeated element: a shared colour, a shared frame, or a shared theme. Mixing subjects works when the palette agrees; mixing frames works when the subjects agree.
Lay the arrangement on the floor first, photograph it, and live with the photo for a day. Then commit — and remember that gallery walls are meant to grow. Leave room for the next print you fall in love with.