@freemo for practical reasons, I reluctantly settled on foot/inch-based formats for my paintings: 18:24 and 24:36 (inches:inches). It doubles the surface area but doesn't preserve the aspect ratio (3:4 vs. 2:3). It's not too bad because if I double the surface area again, I'm back to 36:48 = 3:4. So instead of being always 1/sqrt(2), it alternates between 3:4 and 2:3.