In an ongoing experiment with micro layouts, this one comes in at 32 inches by 40 inches, which is a hollow door picked up at a lumber salvage yard and sawed in half. I was interested in a number of questions, such as: 1. Is bigger better? Is it more or less enjoyable than a layout of a larger size? 2. How small can I go in HO? 3. How will I compress a scene into a small space and retain the essence of the thing?
I think size directly relates to how much you like to detail things and scratchbuild. If you like intricate details, smaller is easier IMO as the scenery and so on takes longer per unit of area, all else being equal.