UV pen doubles up as a torch (when desperate enough). My hand span is exactly 7.5 inches. I used to work in labs, and I'm quite good at estimating sizes to .25 inches accuracy, on big items, but I can do to 2 millimetres accuracy as well, for small things.
But you're right. I do need a tape measure. Like scissors, tweezers and nail files, I have thousands upon thousands (nearly!) of them and cannot lay my hands on any of them!