Then is there a rule of thumb? My drive way has a fence on either side with the opening being 12'. My 22' trailer will go in fine but I'm closing in on the curb on the opposite side. I'd like to get a longer trailer, 29', but am concerned about being able to get in the drive.
You basically have answered your own question.
If your having to hug the opposite curb at the present time, start your turn angle, stop get out and phyiscally measure the distance you have remaining between your current trailers rear and any object that would be a interference, also check clearance from front of truck to any object that would be a interference. Once you have done that subtract another 7 feet from the rear of trailer distance as well as shift the position of the trailer farther out (increase the diamter of the arc of the trailers path "you have incresed the trailer length, arc will be larger" ) You will be able to tell very quickly if you will have enough clearance or if your going to run into problems. You will also find that the position from which you start your turn will have changed in relation to your driveway.