When canvas gets wet and then dries, it shrinks. Sails get wet from rain and from spray, and it was even done on purpose to increase the speed of sailing ships.
For canvas tents, this has to be taken into account when building the frame as otherwise, you can end up with a tent that is too small for it.
Were/are there any similar considerations on sailing ships?
There are no similar issues with sailing ships.
Sails are never designed to fit tightly within any kind of frame. To be effective they must be able to change shape and position, something that is done with lines (called sheets). Most sails are fastened to solid objects only on one or two sides (occasionally three) but in all cases those solid objects can flex or change position relative to each other.
For example the mainsail of a normal single-masted sailboat is attached to the mast at the front, and the boom at the bottom. The boom is pivoted where it attaches to the mast, and also the attachment point can be moved up and down the mast, and the sail can slide up and down the mast and along the boom.. This absorbs strain due to changes in wind pressure (as well as allowing control of the sail shape), and is more than enough to cope with any changes in size due to shrinkage.
