Most of the marinas know the length of boats .. You just got to watch you dont have anything overhanging .. We have a bowsprit that pulls back when not in use so no overhang .. With it out would cost £1.50 a night extra or £200 a year .. /forums/images/graemlins/mad.gif
In Mediterranean France, beam also comes in to the equation because boats are moored end-on, not side-on.
Thus, if you have a 10 meter boat, you might find a variety of berths on offer, 10 x 3.0, 10 x 3.2, 10 x 3.4, etc. The beamier the berth the more expensive it will be. Remember to allow room for fenders, too - although on one side only.