RE: What size?
That english looked pretty good to me. On to the boat question, but first I have a question for you. Are you planning on building from plans, building a kit or buying a finished hull?
As for length, here in the States, we usually run a hull in the range of 77 to 92mm. As far as weight, you want ot keep the boat as light as possible. As impressive as those engine specs seem, the little 3.5cc engines can't really do justice to a heavy boat.