Tamaño (Spanish for “size,” in the size of, “what is your pants size?”) comes from the Latin tam — magno, that is, “so — great” (“great” in the size of “big”). Tam is the Latin for “so” or “very” from which we get the Spanish tan.
To even measure is thus to imply that… you are big! So great! If you’re small, after all, you don’t even need to measure it!
Magno (Latin for “great” or “big”) gives us the English… magnificent. But, curiously, the -gn- turns into the ñ as Latin evolved into Spanish. Thus tan — magno became tamaño. We see this gn to ñ pattern in many words, such as cognate / cuñado.