Ghosts of Saltmarsh provides both stat blocks and deck plans for the several kinds of ships listed in the Dungeon Master's Guide. The stat blocks include, among other things, ship sizes expressed as "X ft. by Y ft." Deck plans, meanwhile, are presented as maps on a standard grid of 5-foot squares.
However, the deck plans don't all match up with the ship sizes stated in the stat blocks. The sailing ship, in particular, is listed in its stat block as having a size of "100 ft. by 20 ft." But its deck plan clearly shows only 16 squares, or 80 ft., from bow to stern.
It is tempting to try to resolve this discrepancy by supposing that a ship, like a creature, might control a space larger than its actual physical size. (See PHB p. 191.) The typical medium humanoid, after all, isn't 5 ft. wide; it's smaller than the space it controls. However, that supposition is undermined by the galley, which is listed as "130 ft. by 20 ft." and yet measures 27 squares, or 135 ft., from bow to stern. It is difficult to imagine how a ship could be larger than the space it controls -- and so the whole notion that ship size is about controlled space rather than actual size appears to break down.
What, then, is the actual length of a sailing ship? Is it 100 ft. as its stat block states, or 80 ft. as shown in its deck plan, or something else?