My opinion:
Length: 14' or less
Beam: 8.5' wide (as much as I agree that wider is better, wider is also more complicated and more work, which goes against what it seems we're really trying to do here. Add wings for a wilder recreational ride, but figure out how to race without them).
Mast length: 24'- these things are nice and light and the extrusions are pretty cheap. No real need for spreaders. I am not against someone showing up with an all-carbon mast, and will gladly race against them with a salvaged H14 stick. Carbon masts may even be relatively cheap to build at these sizes and strengths.
Sail Area: How much sail can you crowd onto a 24' stick? I really wouldn't want to sail a uni-rig if it could be avoided. A roller-furling headsail that could be used up- and down-wind would be great. Something like a really big jib on a pole
Weight: 150-200lbs should cover it, boats could be built in the 100 or less range with exoctic materials that are out of the ability of the average DIY builder-sailor, and out of the price-range of someone just getting into the sport.
Adjustments: I'd give free reign on adjustment ability. I've sailed and raced on Prindle 19s and also on my G-Cat 5.7M. Aside from a few basics, all the extra adjustments just seem to add weight and complexity. A lot of lines to pull would confuse and scare away new people to these boats and this sport. Allow their boats to grow with their skill, and as they get better they can add whatever adjustments they feel they need to.

I would like to comment that I think the Cheshire is an excellent design, but I cannot fathom why it is so narrow (6'5") The cross-beams look like simple mast extrusions and easy enough to replace with wider versions. Also, I'm sure the manufacturer wouldn't mind widening the boat to 8.5' if it meant making the sale.

Also- especially for home-builders, the Quattro 14 design is already out: Quattro 14