You mean units, which are at the end pretty pointless anyways. Each modeller uses his own scale... so maybe its the best (easiest) thing to just scale em at the very end.
You could for example give a rough figure for the dimensions of the ship in... uh... meters. Or you could create a obj (for example) file with 2 cuboids. The big one can be used as guide for the size of the ship and the smaller one is the size of a human... used for getting the scale of the cockpit right.
Apart from that the models should have roughly the same amount of polys. The ships in the first wipEout were *very* low poly. The amount of polygons was so low that they could have been created with a text editor (seriously).
Oh and textures. You should also specify the size.