Car trailer Size?

I have hauled my Dart on a 16' trailer for over 17 years and my 80 Mirada for 5 years at 70+ mile an hour with absolutely no problems. When on the trailer the front tires were about a foot and a half from the front rail and the same for the back tires to the back rail. When centered like that it kept the tongue weight in check. Bought a new 18' aluminum trailer 3 years ago and basically centered the car the same way, tows perfect. Big difference is with the 18' trailer the rear bumper of the Dart is wright at the back rail of the trailer and with the 16' it hung over the back rail. Oh a 12' deck is a 144 inches that makes it just shy of 3 foot longer than the wheel base.