You probably all ready know this one, but for the original poster, this might be new . . .
Typically, you don't want to put anything through the planer that's less than about 12". If you do need to plane something that is less than 12", an easy and safe way to do it is to glue on some side runners that are 12" long. If you've got a piece that's say 6" long and then add two 12" pieces on each of the edges, it might look something like this bit of ASCII art:
+-+ +-+ | | | | | | | | | | | | | |+------+| | | | | | | | | | | | | | | | | | | | | | | |+------+| | | | | | | | | | | | | | +-+ +-+After getting the piece to the desired thickness, just cut off the side runners.