20 foot is he way to go. 16 is too short.
I picked up an 24 ft this summer. Fits my needs perfectly, but could be too much for a 1/2 ton. These are best towed by 3/4 tons, but you might get by with a 1/2 if the 1/2 ton is set-up just right.
The 20 footer will be lighter (since it is smaller) and you can run it lighter since you will put less junk it it (since it is smaller).
The problem with 16 is that is just fits the car with 1 foot to spare on each end. These can actually be harder to tow since you can't as easly dial in tongue weight by moving the car around. Plus with a 16footer you don't have any room for gear. Might as well go open trailer at 16 feet.
Anyway good luck with 20!
Even with a 20 I suggest a load equalizing hitch to manage tongue weights and extended tow mirrors. I got both for my 2500 dodge when I got my 24 footer. The stock mirror were useless with 8.5 wide box and the load equalizing hitch keeps the trailer and truck level.
I got an E2 hitch which also combines some sway control which is a nice feature when you go with a big box. I towed for years with an open trailer without any issues. The big box created a new set of challenges. Now with big tow mirrors and hitch I am set just fine.