I don't see the problem. I agree with Roger, a 68 kg guy can easily uphaul over a 100 lt board, ad with a 5,3 that is the best board for a nomal-intermediate sailor. I'm 90 kg and I easily uphaul over a 112 old board (Mistral Score), even using a slightly out of range Superfreak 8,0.
Obviously if someone wants to have more volume to have more safety for the conditions, and/or for his still low learning level, none forbid to use a bigger board, well knowing that it will be less than perfect. In the first years I rode a Mistral Echo 138 lt even in waves in the sea with NW strong wind in Tunscany, yes it was a rodeo, but I was able to uphaul even in the middle of the sea (yes I also was younger...).
Volume is reated to sails, are the sails that change range of wind and use with different weights.