I've started to experiment recently with Version 0.99.6a (SVN Rev 22276) and WinPcap version 4.0.1 which was the recommended version when I installed Wireshark. As far as I'm aware, ethernet frames should be between 64 and 1518 bytes long and, if the data section is less than 46 bytes, padding should be added to make up the minimum length. Further, I believe that this minimum length is something to do with collisions.