12" is about as short as you would probably would want to use. There are techniques to make "runners" for short pieces so they can safely pass through the planer. As for thickness, most planers suggest 1/8" as the minimum thickness. You can make an insert to rest on the original bed. This will allow you to make thinner stock. When planing material thinner than 1/8", it will have the tendency to shatter- especially in areas where the grain runs wild.