Also look at what size is most commonly available.
Not sure what a Prima is, but the Racor
500 series is I believe the most commonly available size. Itís much larger than needed for most sailboat engines, but availability of spares canít be overstated in importance.
If you know your going where fuel
is sometimes questionable, a dual filter setup may be a good idea, not inexpensive though, and buy at least a case of filters.