Hey mate,
Maybe Huey from Huetts has a simple answer - as he is the Etec specialist, but in my experience its not a question you can answer easily. It comes down to a bit of trial and error due to the specific nature of your boat and how many people and gear its carrying. It also depends on the material (stainless vs aluminium) and whether you want 3 or 4 blades.
What I can say from experience is that your Etec should be putting out 5500-6000rpm at WOT with a 'normal' load. 'Normal' being the usual number of crew and gear. If its not getting that then you are probably swinging too big a prop (and the engine is struggling to cope) , and if its getting more then it may be too small (and you are overreving the engine obviously).
One thing I remember everyone telling me is that you should try to ensure your motor is fixed at the right height before looking into the prop performance. I've heard that incorrectly mounted motors can make for the biggest performance issues. My understanding is that the cavitation plate (just above the prop) should be skimming the water at WOT. If its not, then you need to raise or lower the motor mounting before you look at props.
Once you are set there, then you can start playing around with props.
For comparison, I have a 5.3m Quintrex freedom sport - My F115 came with a 15 x 17 inch pitch 3 blade stainless, and I was only getting about 5300 rpm at WOT. I wanted a 4 blade, because I was told that you get better fuel economy and torque at low speeds, but sacrifice top end speed (which isnt an issue for me, I'm Mr McGoo on the water). I also chose stainless because I was told that performance is better (and it looks better!). I was advised to get a 4 blade 13.25 x 15 , which I must admit I thought sounded ridiculous - but the extra blade adds a lot of drag, so it does make sense. The advice I got was spot on, and I now get 6000rpm on the knocker at WOT.
I hope thats at least an interesting read. Best thing though would be to get some professional advice from Huey, once you know what your RPM at WOT is, and you are sure the motor is mounted right.
regards
Fryboy