The standard size for presentation folders is 9x12. This size is optimized to hold standard letter (8.5"x11") documents and will fit easily into a 10x13 envelope if mailing is required. The second standard size is 9x14.5, designed for standard legal (8.5"x14") documents. These two sizes will handle most, if not all, of normal business documentation, and come with 4" pockets.
Mini folders are also a frequent request, and the three most common sizes are 6"x9", 5"x9", and 4"x9". These smaller sizes have 3" pockets and are intended for smaller documents like brochures or receipts, among others.