termites - it's ridiculous because it varies a lot, but lets say that up to 24 termites is how many variations there are.
a termite would be likely juuust under an inch, to just over a centimetre in length.
for example, my laptop would probably be 13 or 14 termites in terms of screen size, but it's actually 12.5" in screen size, lol
the "termite ruler" would be made of metal since... well, it'd been eaten already if it were wood. anyways, it's about 24 termites long, and it goes up from the shortest to the longest.
imagine being taught this in schools though lmao-
edit: the symbol would be Tm or TM, depending on location and dialect.
like 100Tm for example