Go as big as you can afford and pull. 500 dollars isn't much in the grand scheme of things, considering you're paying 2 grand for the trailer already! I really like my ramp door, it sure makes it nice to walk in and out. My brother has the same trailer, but decided to save the 200 or so and just get the regular doors in back. He likes his just fine too, so whatever way you decide on a back door would be fine.
I'll send you some pics later on when I get home. I'll include some measurements as well!
This trailer pulls real nice, and is very light. The tongue weight is only about 120 lbs, unloaded, and the trailer is just over 1000 lbs unloaded, but it does catch a lot of wind going down the road. My van pulls it with ease, but I'd rather pull it with my pickup given a choice!