If you're dead set on using mecanum wheels, but still want to be in the "small wheel" category of offensive teams, these may be if use. If not, then you can probably use a 3D printer to print out some 4" ones. I've heard 3D printer wheels can actually be quite strong if you use the right material.

All in all, a good 6wd, drop center wheel will be the best way to get your team headed in the right direction. Why? 1 word: Traction. You don't know how useful it is until you're without it.