The best size car trailer for you is highly dependent on what you're going to tow. If you've got an old Mini Cooper or Torana race car you are going to be towing to different places, then something small will be easier to build, and store. So the size of the trailer is highly dependent on what you want to tow.
The weight capacity of the trailer is also something that you need to consider. If you plan on towing larger 4WD's you will need to build a trailer with a much stronger frame.