Teardrops n Tiny Travel Trailers

I am going to build a 5' x 8' frame. What size steel tubing do you recommend? I was thinking about
2 1/2" or 2 1/4". I am also thinking about going to a place that makes tin and buying 5' long flat pieces and using them for the top. I had planned to overlap these pieces about 2". Should I put a underlayment such as 1/4" plywood under the tin or just use tin with 2" x 2" strips? I will use 3/4" plywood for the sides and floor. Any advice would be appreciated.

My frame is very simple, it is made from 2x2x1/8. I felt that the tongue was weak and it was replaced with 2x4x1/4... way overkill. I wanted 2x3x3/16, but they couldn't get it on short notice.

I think you ought to find out what sizes are most likely to be readily available for your build. Not all steel suppliers will have a complete selection of tubing. Most common....the 2x material. I've used it in .125 down to 16 Ga. Doug