Because I entered this pretty much blind as well, I did a lot of reading and studying prop charts from both the engine and prop manf's. Then I took a WAG/Wild Axx Guess based on my findings, ending up with a 13 3/4 x13. Lightly loaded (no furniture) the prop is just a hair small. Target rpm was 6k, and we ended up at 6.3k running 25-26 mph. Normally, I would have kept that prop for a spare and gone with the next size bigger to get the rpms down under 6k. Here though, I need to see what it does when I finally get my furniture. Thinking that extra weight may bring me right where I want to be. I did end up with a Honda OEM prop I bought new for less than 100. shipped. Point being you need to shop around. A local dealer tried to charge me twice that much....
So a guess on a 225hp engine? Geesh! I don't want to make a call that's going to cost you a lot of money if I'm wrong. I will share the fact my studying could not justify a stainless prop. With more than twice the power though? I'm not sure that would hold true any longer. I think I would do my experimenting with alum props at first though, and when I got close, maybe go SS at that point.