I'm using 16 gauge cold rolled steel for my sides, but I have a bit more or a compact box, with more support beams. I will probably go with 14 gauge for the flooring (only 6' long by 4' wide).
Because I'm breaking mine up into compartments I have four 1" tubes that breaks my sides into 3 compartments. I didn't bother going with a large sheet to cover since I'll have doors that open outward and then will have additional 1" tube frames to add extra support, so I don't need all the strength in a 14 gauge or 11 gauge. ( I have resolved myself to avoid aluminum after I saw a buddies pride and joy trailer get his door bent just from brushing past it.
I would say, if you're going to use aluminum, make sure none of your door hinges are aluminum.
Here's a link to my build.
http://www.expeditionportal.com/for...ot-in-the-budget-Custom-Exp-Trailer-to-follow