For 18x24 I use 25x36 minimum, and honestly that can be a bit of problem on some art. What I have always read, and it seems to hold true in practice is you want the image to be 2-3 inches from the inside of the frame. Most frames are around 1.5-2 inches square, so lets say on a 23x31 you would end up with a max width of around 16 inches and height of 24. Ideally you want more space though or straight lines will begin to distort in the corners.