Neil,
I weight 74kg and I do own a Stype 104 the sail sizes that i used on the stype was from 4.8 up to 6.4 sails..
I even use this board during the PWA Gra Canaria slalom42 dscipline with a 4.7 and it was working great in those high wind even jibing perfect.
In Costa Brava spain i use this board with my 6.4 switch mauisails and the board was also okay.
I wo't recommend going bigger then 7.0.. but the board is pretty fast and sure you will enjoy it.
Taty