My 5er is currently not at home but the last time I towed it I noticed my lube plate is pretty much done. I need to order a new one but notice there's a 10" and 12" plate. I didn't measure my current ...
If it sticks too far out the front of the pinbox, you are likely to break it when the pinbox is climbing the fifthwheel plate. So for that reason you don't want one that is too large. I don't see any harm in using a small one, so I think it is best to just stick with the smallest you can find.