Many varieties will grow to their full length and then start getting wider when they are ready to pick. I grow vegetables close together (to try more varieties -- at least a dozen types of cucumbers this year), so my vegetables sometimes don't get as big as they're supposed to. Trial and error is a good method, and it will train your eye. If a cucumber "looks" ready, try it. Does it taste right? Sometimes I pick a cucumber a little too early because I can't wait (or won't be able to get to the garden for the next day or two), and it's obvious that it could have stayed on the vine a day or two longer. It's still edible! And it's better than letting it get overripe.

You can even take daily photos of your developing cukes so you'll have a visual record of how long they take to ripen (which also depends on temperature and watering), and what they look like a day or two before they're ready.