Trailers weighing under 1500 lbs. and under receive motorcycle-sized plates as shown below.
Both types of trailer plates are quite common and can be easily seen in use.

1986

1991

1992

1998

The A00000 format is still in use today, and should last quite a while.

1999

2006

2009

This is the only trailer vanity I've seen on the small size plate.