What is the length of one symbol sent through WiFI?
You can download the 802.11-2016 standard for free from IEEE (it has been published more than 6 months ago, so IEEE allows it to be downloaded for free, see https://ieeexplore.ieee.org/browse/standards/get-program/page/series?id=68) and look through the numbers in there.

But you're looking for, I guess, 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac? No problem. All these amendments have now been approved and rolled back into the 802.11-2016 standard (the 2016 means that revision of the standard; every few years, a new version comes out, where some of the latest approved amendments are rolled back into the standard). But will the physical layers specs for each of these amendments all be mixed together and hard to separate? No, they are neatly separated into different sections, so the numbers are not hard to find.