I don't know but I've wondered this too. I have one that is a short flap but it appears that the block is the same size as the regular flap (or no noticeable difference to the naked eye). When I ordered the saddle I wasn't offered any block options or anything else, just flap length. Fitter saw that regular was a bit too long.
Relatively smaller, ie the issue CC mentions where the block is the same whatever the flap size (panel is the bit with flocking or foam or air in it ), or they actually measure smaller? If the latter you'd need to ask Fairfax.