Lengthening the trailer tongue is a different matter from lengthening the
distance from the ball on the ball mount to the tow vehicle.

In the former, the center of gravity of the boat/trailer is farther from the
center of gravity of the tow vehicle, but that by itself doesn't make much
difference in towing and handling (all else being equal).

However, if the distance from the tow vehicle to the ball mount is longer,
any sway in the trailer has much more of an effect on the handling of the
tow vehicle. Think of a lever and fulcrum. The longer the lever from the
fulcrum, the more movements are magnified.

Common lengths, from the hitch pin hole to the ball hole, are around 8
inches, with the longest I have commonly seen about 11 inches (and the
shortest something like 6 inches). Not all hitches have the same distance
from the back end of the hitch to the end of the vewhicle (bumper...), and,
of course, not all vehicles have the same distance from the center of
gravity or the rear axle to the hitch. Therefore, I would imagine that the
length of the ball mount in the ranges commonly available will not have much
effect on towing. The most important factor is probably tight turns, with
the trailer not running into the tow vehicle (pretty hard to do with a
standard pop-up configuration!).

Note that there are hitch ball mount 'extenders' available in lengths up to
18". These usually state in the fine print that the maximum towed weight
should be reduced by 1/3 if using these, and I would also imagine that any
sway in the trailer could be dangerously amplified.

The welder who modified my power boat trailer into a sailboat trailer,
used a bathroom scale to see how long the trailer tongue had to be to
give a downward force of 200 pounds on the ball. He had to add 18
inches to the trailer tongue to get the rig to balance out but it was
really worth the effort and cost because the trailer & sailboat now
pull very well. The trailer tracks like it was on rails and the 18
inch addition means that, even in a very tight turn, the boat and tow
car never touch. In addition, the rig even backs much better than
previously.