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 ...
With our mid-range 5r using a standard kingpin/box and a Reese 16K hitch, I use a 10" plate. Seems to work just fine. As the edges of the 10" sticks out a bit, I think the edges of the 12" would stick out too much and be susceptible to bending or breaking.