The U (rack unit) is the standardized height unit for 19-inch equipment: 1 U = 44.45 mm (1.75″). A standard rack offers 42 U of usable space; newer facilities often provide 45, 47 or even 52 U.
Sizing rules of thumb: a server is 1 U or 2 U, a switch 1 U, a firewall 1 U, a fiber patch tray 1 U, a NAS 2–4 U. Always keep a 20–30% growth margin, and check the two constraints that saturate before height: depth (modern servers exceed 750 mm, so the rack should be 1,000–1,200 mm deep) and above all power (kVA): a rack full of recent servers hits its electrical ceiling well before its last U.