Non-commercial trailer plates are a bit smaller than the standard plate, and their proportional sizes are a little different. The motorcycle plates appear to be the same size.
I've seen a few references state that the PH plates (see below) are for larger non-commercial trailers. The distinction seems to be a littler more precise than this. Large Winnebago- style trailers often appear with the smaller trailer plate. I've spotted the larger plates only on trailers that weren't very long, were completely enclosed, and were relatively tall (as tall as the cab of a pickup, generally.)
I also have myself a full-size P-series plate ([8981 PB], below) that may or may not be an authentic plate.
The R series is relatively recent, assumably because the P series ran out.
Missing photos: [RE] [RF] [RG] [RJ] [RK] [RL] [RM] more?
These plates are now also being issued to motorcycles.
Missing: [SG] [SH] [SJ]
I haven't even seen an SB-suffixed plate; it seems that that suffix was deliberately omitted.