sorry it's taken so long from initial contact but here's my tupence worth......
Everything you have been told so far is correct. You do need to know what RPM your current prop is making and you need to know what Mariner says your WOT (Wide Open Throttle) revs should be. Altering the pitch of the prop will increase or decrease the rpm as stated by xs400 above. This in turn will allow you to choose the correct pitch for your new prop. You should ensure that you boat is loaded as for 'normal' use (which is easier said than done if you use your boat in the same manner that I used to use mine. 'Nomal' could be anything from me on my own going fishing to four up exploring to towing a doughnut / wakeboard).
In answer to your question to me I have two props that I used on my 3.8mtr Zodiac Futura S / Yamaha 30HP 2 stroke combo. 9 7/8 *12 and 9 7/8 *14 which I swapped depending on what I was doing. In practice you should always carry a spare prop anyway
. I suggest that you do not simply plump for the same pitch as myself or xs but work out from the above what you need. If you need a second opinion once you have your current prop rpm and your Mariner WOT operating rpm post it up and we'll have another chat.
Having said all the above, hieght of the engine on the transom (and therfore cav plate position to the water), how you load your boat (how much wieght you have in the bow either people or equipment (fuel tanks / anchors etc)) and as you have already illuded to trim all make a huge difference. If you have power trim you should trim all the way in for take off and then trim up for top speed. This of course is not possible with manual trim so you have to compromise a little. I used to use the second hole up on my manual trim Yam 30 which seemed to give the best overall results. It's difficult to know what else to suggest without knowing the specifics of your set up. You could post pics of engine position which might be helpful. Give me a shout if there is anything else and I'll try and help if I know the answer.
Sorry if you already know about all the above but I seemed to have gone into download mode