Wingspan Limit
I asked the same question, in a different way, some time ago. The conclusion I drew is that, although IMAC has no official size preference, it's really all about big airplanes. Virtually all of the active participants in my region fly big. Even in basic.
Mini-mac is essentially dead as far as I can tell. I think it's a good idea with obsolete rules.
The best suggestion was club-mac or club level IMAC. This idea is intended to be workable at your own local club several times a year. It would encourage people who would never even consider IMAC otherwise. There would be an airplane size limit to prevent the real or imagined intimidation factor and cost impact that you can get at a regular IMAC event.
In the Northwest region, there are 12 events this year. If you went to all of them, you would log some serious miles driving. People like me can simply not afford the time to drive 3 hours to an event, spend the night, and drive 3 hours back. All this for 6 to 8 flights lasting 3 minutes. On the other hand, a club-mac format at my own club or one of the other locals would be very attractive. Just another day at the field with some sequence flying and judging. A real peer oriented event.