If you are set on the idea,something about 4 to 4.5 metres with a windscreen and canopy with sufficient horsepower to avoid needing to run at high revs. A tinny won't need as much horsepower as a fibreglass rig and will be a bit more economical but will ride a bit harsher in chop. To give you an idea on fuel burn, both my 15 foot tinny with a 40 and my old man's glass 15 footer with a 70 use about 25 litres on a round trip to the pin from redland bay. Both are/were 2 stroke engines.