Yes, since by just thickening the steel girder (keel, any other part) you're not gonna get linear cost efficiency. Double the thickness won't mean double the durability.
that being said, theoretical limit to longest/largest single hull ship may be unknown... and is certainly somewhere way, way up there past the mile range. Trouble is, percentage of usable ships space would be much lower on such a mammoth, precisely because of all the reinforcements needed for such huge strains. And it'd still be too expensive to justify the gains.
there is a reason why military ships are sticking to certain sizes, while commercial ships can still afford to go bigger and longer. They're just not efficient.
