First thing I'd look at which nobody else has mentioned yet
is the Max HP rating for the boat on the transom plaque.
Then decide between 2 or 4 stroke. That's how I'd approach
this - I wouldn't necessarily go with the biggest motor the
boat is rated for either.

Thanks for mentioning about the max hp rating of the boat, and I will
not likely to go for the max either.

I will likely also take into account of the range of motor size that
the manufacturer has bundled in their boats. If they are offering the
same boat with both 90HP and 115HP. I will need to make sure I will not
go below their range of motor sizes too far, or better yet just stick
with their minimum motor size (in this example, that will be 90HP).

Jay Chan