By Marion Boddy-Evans
Though most people do use identical sizes there’s no reason why you must, no set convention. Do what feels right for the painting. I once did a triptych which was to hang on a wall alongside a staircase that was open to the living space of the house. I painted it on with three different sized canvases painted so the scene aligned at the top.
Traditional altar-piece triptyches for churches used to have one large one in the centre and two smaller ones on the sides (same height, different width). Some had the smaller pieces hinged to the centre piece, and these could fold over to cover it and had different paintings on the back.