Frank Leonard Sizer (September 29, 1856 - July 11, 1942) was an American civil and mining engineer active at various sites. He was born Ottawa, Illinois, and studied engineering and surveying at the University of Michigan. On graduating, he moved to Leadville, Colorado, then in the early days of the Colorado Silver Boom (1879 - 1893). He stayed there for several years, mapping mining claims, before relocated to Helena Montana Territory by March of 1882. There he was both a chief clerk in the surveyor general's office, and maintained private business interests. He was a partner in Sizer and Keerl (with James S. Keerl), a Colorado mining concern active from 1890 - 1897. In 1901 he spent a year consulting at the Rosario Mine in Chihuahua, Mexico. He participated at various levels in numerous other mines throughout Montana and was active in various trade organizations. In 1909, he moved to California where in 1942 he died at his home in Berkeley.