From the marina's point of view, what matters is how much dock or slip length can't be rented to someone else. What the boater is paying for is the exclusive use of space.
Nobody is fond of a Scrooge (except Scrooge's stockholders and, I dunno, maybe Scrooge's mom). On the other hand, as a marina tenant myself, I'd rather have the owner be fairly strict on measuring than have him let someone put 28 feet of boat in the 24 foot slip just across the narrow fairway from my boat! On a windy day with the tide running strong, it can be nerve-wracking enough trying to get in or out of a slip when the fairway is as wide as it's supposed to be. The added fun of a fairway narrowed by protruding sharp pointing things like anchors or skegs really puts my knickers in a twist. The same can be true at the walkway end of slips when over-length boats are snugged in as far as possible leaving a gauntlet of bows to weave through with a dock cart. Especially in a tough economy, some marina owners are understandably loath to turn away any potential paying tenant just because his boat is a "little" too long for the only available slips.
I don't know what the configuration is of the mooring space you're hoping to find, or whether you're seeking wet or dry storage, for that matter, but sure hope you can find a reasonable and fair operator.