You'll need to give a bit more info. eg exact boat, loading, the speed that you expect?
I had a Seapro 520 with a Merc 75ELPTO. I ran that with a 18" Vengence prop that gave exactly 40 mph with four adults and around 50 litres of fuel and would pull two light skiers with two adults in the boat. It could have been pushed to 20" for "one-up" absolute top end, but the Seapro is quite a light boat and I'm guessing, unless it's something very similar, 17"/18" is probably what you'll need.