Remember the weight rating of a trailer, GVWR, is the weight of the trailer and the load it carries. If buying a trailer to haul cars I would think you would want to get one that would haul the largest car you might expect to have and need to move. That is what I did in my case.
My truck is a Ford F250 Super Duty Crew Cab which weights somewhere around 7000# with fuel and other excessories such as tools. I used that vehicle as the maximum vehicle size I may move, and it also was convenient that my tractor also weight somewhere around 7000#. The truck is also a little over 20ft long which meant a 20ft trailer. Now in my research I found that 20ft trailers weighed somewhere around 3000# so adding my trucks weight, or tractor, at 7000# the GVWR would need to be somewhere north of 10,000#. I went with a 14000# GVWR trailer which give some additional weight capacity for added safety and the cost of a 14000# vs 10000# trailer was minimal compared to the total cost.
This of course was what is needed to fit my needs and your needs may be different but I would suggest you go through the same thought process I did to determine the best size to fit your needs now and the future.
