It may vary by state, but generally it is 3500# gross. That would normally be a single axle trailer. Generally the lowest tandem axle GVWR is 7000#, which would have two of those 3500# axles and would require brakes. Some of the older tandem trailers did not have brakes, so you have to be careful if shopping for an older used small tandem trailer. In my state of Kansas, trailers with a gross actual weight of less than 2000# do not require a license, but it is that same 3500# trailer and people forget that by the time you get it loaded, it exceeds that 2000# limit. Very few of them running down the road are actually legal.
Dave