Mite is one of many words for a very small amount, like a bit and a jot. A mite is teeny-weeny and itty-bitty. It's not much at all.
Put away your ruler, because you can't measure a mite any more than you can measure a smidge, a tad, a speck, a tough, or a pinch — all English words for a tiny amount. Such words are useful when making polite requests. If someone's just barely blocking your car, you might ask them to move theirs just a mite. If you say you'll be back in just a mite, you won't keep the person waiting long.