Park Tool RR-12 tape measure
Buy one of these handy little devices and stash it somewhere for safe keeping (but don't hide it someplace dumb where you'll forget about it).
Then, when you email us in s a few months saying you want to buy a new frame but don't know which size to get and we respond instructing you to take some measurements of your current frame, you won't be able to tell us you have no way to do that.
• durable composite housing w/ rubber grippy areas and belt clip
• provides both English and metric readings up to 12 feet (3.65 m)
• works on all kinds of stuff (not just frames)