http://www.pacific-recycling.com/SStube.html
This chart shows weights per linear foot for round, square, and rectangular steel tubing.
For 11 gauge 2 x 2 square tubing, it shows 3.07 pounds per running foot.
For 11 guage 1 x 3 rectangular, it shows it also weighs 3.07 pounds.
Of course, that means a 20 length of 2 x 2 square tube steel will weigh 61.4 pounds. This would be helpful in figuring your basic frame for the trailer without weighing.