I wonder how many people with bigger enclosed trailers have safety chains that meet the defined "each safety chain having an ultimate strength at least equal to the gross weight of the towed vehicles"? Take a look at chain ratings here:
http://www.1st-chainsupply.com/WLLchart.htm
It looks like anyone with a bigger trailer needs two 5/8" chains to be legal!
On edit (I'll blame it on the cold medicine): Looks like ultimate strength is 4x WLL for Grade 70 chain, so 5/16" Grade 70 should suffice (I have 3/8" Gr 70 on mine now).
My trailer (16k GTWR) came with 1/4" chain held on with cold-shut links. Trying to pull the trailer over a hump in my driveway, the coupler broke loose. Fortunately the trailer didn't roll as it was hung up on the driveway, but the cold-shut links broke so easily I didn't even feel it in the truck. I discovered at that point that my breakaway switch was corroded and had quit working as well. From now on I check the breakaway switch monthly during racing season.