First decide how long you want this trailer to last. If 2-4 years then a cheaper trailer with plywood inside will be okay. If 5 years or more then go with an aluminum trailer. Not for gas savings but for longevity and resale.
20' and 24' are popular sizes.
Add an electric front jack. It will make life easier. Get 12v lights inside.
The trailer floor height will dictate how much of a fold out ramp extension you will need. This is important or you will be stacking 2" x10" boards to load/unload.
Add at least 1 roof vent.
Do you use or need a small generator? If so add a generator access door.
You can use a 12v winch and rear tie downs and load a car without getting inside the car. This can help if you can't get the car door to clear the trailer wheel well.
But, your car front wheels must fit inside the trailer wheel wells so make sure the trailer is wide enough inside.
I agree with the others. Avoid any trailer with 5 lug wheels. The lugs will sheer off and it's an indicator that everything on it is light duty.
Like everything these days quality trailers are in short supply. Allow plenty of time to research and find the right one. If you want to see the best, look at Gold Rush.
http://www.goldrushinc.com/
Good luck!