Peter has been really busy so that one slipped through for now. Website is going to get some more updates and info/description of the designs.
We made the 53 wide as the biggest speed board, because if you need any more volume than that the slalom boards are practically as fast in the lighter conditions.
They are optimised for flatter water- a bit thicker in the rails for easier starting, and you *can* use bigger sails on these narrower boards due to the bottom shape and rocker: really easy starting and control, but trading ease of gybing in wild open ocean swell and chop (ie where nobody would ever try to use a speed board anyway). Theres always a tradeoff, but this was a no-brainer.
I've sailed them in some really rough water and they are still a joy to ride. On flat water they will be alpha machines (just waiting for some flat water)- I cranked a few gybes in moderate chop and its got that super tight fast in- fast out feel.
Main advantage of running these smaller boards is you can run smaller fins for the wind conditions and sail size, and still feel balanced. The ride is really dry- its surprisingly easy to get planing, and going through any chop there is no noticable spray, or catching of the edges or nose. Pretty cool feeling!
I tried the 53 and 48 with my 7m the other week in an easterly at Pt walter on a particularly choppy day, and was able to go full tilt, no problems in the middle of the river, with no issues getting back upwind. Hardly speed conditions. Sail did not feel too big for the boards which is unreal. Normally I don't sail speed boards on the river but I think I'll make an exception for these ones

The guys in Europe have had some proper wind to test the smaller ones in crazy rough conditions- and they are smoking with the smaller sails too.
So on the sail sizes, you could probably get away with a 7.8m on the 53 at a stretch, but 7m feels pretty good to me. I'll be using it down to my smallest sail (5.7m). The 48 felt OK on the 7m, but if I'm on that sail size I'd just use the 53.