http://privexioc67u24lsmssoeixnml2exr3les4pbtyqtmv3zvonvcc72jyd.onion/articles/suggested-switches-routers-colo
Please see the following page written by a third-party (unaffiliated with Privex), which explains rack units in more detail, and has a table showing the measurements for each Rack Unit size: What is 1U, 2U, 3U, 4U, 5U, 6U, and 7U?