it is actually a very good rule, the measure overall width. It is done in many other classes as well. It is a up to the designer to decide to make the cross beams wider or to add wings.
I don't like the wings, because you could fall onto it during a capsize, but that's just my personal opinion.

By the way with a F16, you have alread a width to length ratio of 1:2, as a Tornado.

Cheers,

Klaus