To me, a reasonable takana would look something like this:
(1) Invite all relatives for sit down up to first cousins. This takes care of the major issue that people throw up as to why a fixed maximum doesn’t work – “well what if they have a large family?”.
(2) Invite up to 50 (or some other fixed number) non-relative guests for sit down. If you can’t satisfy this requirement without getting into hot water then invite NO ONE in this category for sit down.
(3) Everyone else to join for shmorg, chuppah and dancing.
Simple formula with least number of machlokahs.