Next, over the approximately next three (3) months the mirrors, constructed of Beryllium (Be2+, atomic number [#] 4) metal, will be painstakingly aligned to ideal perfection with micron (micrometer / 10^-6 meter) and nanometer (10^-9 meter) movements of the eighteen (18) hexagonal (six-sided) mirrors, each 1.32 meters (4.3 feet) in diameter. Altogether the mirror array will have a diameter of 6.5 meters or 21.3 feet (21 feet and 4 inches).

https://webb.nasa.gov/content/observatory/ote/mirrors/index.html