Hicklawns advice is solid. I went with the Tandem as well which is a little overkill, but I have already hauled loads that would have been too much for a single axle. Only thing that I maybe wished was that I went with a V nose, but they are more money. I went with Carmate and they are a well made trailer. There are cheaper ones out there, but when you look at them they are cheaper made too. Like most things you get what you pay for. I went 7x14 which actually inside is 6'4" X 13'8". I think I got an additional 6" in height with mine.
I had a utility trailer but the Cargo trailer is so much nicer especially when going to camp. I can just throw a bunch of stuff in it and don't have to worry about securing or the stuff getting wet.
Like Giles said I looked for used, but the used market was pretty expensive, so I went new.