Different people, different perspectives.
When I am asked about a utility trailer, I picture something full width of the vehicle, and 12'- 16' long.
It is apparent by the posts here, that some people picture something 4' wide and 5'- 8' long.
Since the OP didn't specify, I guess we shouldn't work off our assumptions.
If I was building a small trailer, I'd have removable sides (because it is more likely to haul lawn mowers, gravel, or brush). Plus, the sides are manageable, so they might get used.
If I was building a larger trailer, I'd have a flatbed (because it is more likely to haul sheet goods, scrap steel, cars or car parts). I'd include stake pockets, but I'm pretty sure I'd never use them.