I use both 8x10s and 11x14s, mounted on 11x14 and 16x20 inch boards respectively. The main issue with a portfolio is ease of handling. Anyone looking at it will probably be holding the prints and looking at them closely, so you don't want anything to be unwieldy or inconvenient. Generally, an 8x10 is quite enough to check a photographer's technical quality and skill, but an 11x14 has a bit more "oomph" to it and shows good TQ even better. Anything bigger, though, would be hard to handle.