For a fun day I would probably just say about 7.8 as most people have these. This would be just to get some new guys down for a race and to keep the fleet a bit more even. (Leo, Simon and Cam could be restricted to a 3.7 though

)
For more serious racing, it would of course be the 9.5m Raceboard limit.
The good thing about longboards I think is that the course setting is not critical as it is with slalom. For slalom the marks and the start have to be pretty much exact but with longboards we should be able to set up a makeshift course pretty much by using existing marks etc. ...and for the fun days even a beach start would suffice. This should also make it easier to have more races.
...but these are just some thoughts at this stage. It will ultimately be up to what the members want.