8.5x20 is huge for an F150. It's not the weight it's the frontal area. I would not go larger than 7x16.
There is a cargo trailer conversion forum at tnttt.com. I even have a thread there for my second build...a 6x12 Vee Cargo to ToyHauler under sdiver68. That forum will be your best resource if you build one up.
It's a fun and relatively easy project but it is a project. I estimate at least 120 hours into mine. I did not do water...just partial insulation, heat, AC and room for 3 bikes/gear and 2 sleepers, 1 bike/gear and 4 sleepers, or bikes outside and 6 sleepers. I have some cool features like a loft bed, mounts for wheels on the wall, trim, ability to use any chock or axle stand available, etc..
Last edited by sdiver68; 11-10-2015 at 02:09 PM.