RE: gp extra 330l
no there's no such rule in the BMFA and im sure in the CAA aswell.
The only rule is 20kg cannot fly publically (might be at all now) without CAA approval.
Stuart Mackay's 47% revolution is a beast but its extremely light, not far over 20kg, yet his sukhoi of 35% is the same weight and loads smaller...
its all weight not size.