I don't see how a 16 footer would maneuver better than a 12 footer in some backing situations. You'd have to allow yourself more room for the longer trailer. I've been in many tight situations with my 12 footer that I've been glad to have it instead of a longer trailer. I feel that 12' long is a really nice maneuverable length. It's also nice to have a tandem axle trailer since these will trail behind your truck much more smoothly without bouncing around as much, and not to mention can carry more weight. You'll want brakes at least on one axle on the trailer to be technically legal.
I do agree that you should get a large enough trailer to allow you to expand. You wont be able to get anything on there but your mower with a 5X8. You might buy a wider mower in the future, and you'll want a lot more room to put stuff.
My trailer, which is the one that I started out with, is a 6.5' X 12'. Sometimes I've wished for a 14', but never anything that much longer unless I wanted 3 mowers on one trailer. the 12' fits 2 mowers comfortably as well as my 21" push mower. If you plan on getting another mower with all your big lawns as you expand, you'll be savin yourself money and time in the future by getting the larger trailer now.
The Mean Green Cut
Kevin Rumsey, Owner