I went through this when I repowered with greater hp several years ago.
My original configuration had me getting to a comfortable cruising speed of 7.5k at 2700 RPM on props that were 19x13 with a 2:1 reduction gear.
I changed nothing but the hp and found that the speed at 2700 RPM increased. My fuel economy (mpg) improved about 10% due to the more modern engine.
Then I looked at prop curves, Boatdiesel.com has a good prop calculator, which I used.
I took my props to Osborne in Vancouver and followed their advice, so I now have 19x17s, 8k is achieved at 2000 RPM and my fuel economy (mpg) has improved another 5%.
All this was done in 2000 and I am still very happy with the outcome.