As they say, size doesn't matter.
No, frankly, I just tried a BRDM2-Konkurs with size 1, 2, 3 and the price stayed the same.
On the other hand, the same with FC=25, RF=22 (LRF), stab=3 jumped from 241 to 322. Haven't have time to test ingame yet.
Size 2 will make them harder to see... Are you sure you want that? And remember that the size rating encompasses the launch flash, which explains that missile teams have a higher size rating than snipers.
You talk about size=3, for which unit precisely?
Look out for the relative scaling of vehicles, lest you end up with size=0 vehicles or worse!