Not sure what you're using for a tow rig, or if that is a limiting factor at all, but I'd recommend going a little bigger than what you think you need. I know the purpose is for bikes, but it always seems that between those biking trips, your trailer will end up getting used for moving furniture, building materials, etc... which is great, since it makes justifying the purchase easier.
With the shorter lengths you've described, they'll be either 6' or 7' wide (not actual interior dimension), which is fine. I've always liked 8' wide trailers, since you can fit bikes sideways and not have wheel chocks to trip over. You can also sleep inside the same direction if you use it for camping. However, 8' wide trailers (under 14') are very rare. When I came across an 8' x 12' with a 7' interior height, I jumped on it. Those dimensions make it funky looking, but whatev.
Anyway, like people have said already, get one that you can stand up in. Ramp doors are nice, but not a big deal if using it for bikes, so I wouldn't pass up a good used one if it has the barn doors. If you use it for sleeping in, make sure it has good ventilation by way of roof vent or slider window (small window(s) that someone can't crawl through). Get a v-nose if you can (no reason not to, and it should help with mileage). I've got an aerodynamic bubble thing on mine, but I'm not convinced it makes a significant difference.