What Christyleigh said is correct.
Yes, I sometimes go in to the back room and drool on it. I have considered leaving on the coffee table as "modern art", too.
Back to business: You should see the power curve in your documentation and there are several resources that will tell you the proper prop size and pitch. Part of this is related to the engine shaft output, and part to the geometry of the boat, the maximum shaft size, the aperture or space in which the blades arc (rule of thumb is 10% clearance minimum, so a 12 inch prop would need 1.2" of gap between prop edge and boat hull) and so on.
Dave Gerr has at least one book out on the whole subject and the manufacturers will also give suggestions because it is their interest to sell you the appropriate prop for your usage. There are also web sites devoted to arcane formulas on the topic.
The "gold standard" installation for me, with a full keel steel boat intended for passage-making, is a universal coupler for the shaft-engine connection and "soft mounts" for the engine itself.
While this involves the welding in of a thrust bearing, it eliminates a lot of noise, vibration and all alignment issues. It's not cheap, but it replicates what a lot of commercial boats have and makes sense for me.
For a coastal cruiser, a Drive saver-style coupling "cushion" is sufficient.