Patrick McAfee was given a special tape measure at the Festival of Stone we ran in Montreal last October. He brought it with him to California. We use it to measure the heights of various stones we will need for specific gaps in the wall were building.
Numbers are easy to forget. (Was that three and one quarter inches or one and three quarter inches?) . On other hand as long as you keep your thumb on the tape at the length you need, you can easily walk around the pile, placing it up to the stones until you find the right size one. That way it really doesn't matter whether they're 6 or 7 or even 8 inches, as long as you find the ones that fit.