Fly hook size charts are available on Killroys.com and DimensionsInfo.com. Killroys.com provides information on the materials that constitute fly hooks, such as carbon, steel and stainless steel.
Killroys.com also has information on how to measure a hook; a fly hook comparison chart for brands such as Mustad, Daiichi, Orvis and Partridge; fly tying basics and barbed versus barbless fly fishing hooks. Charts can be downloaded and printed.
DimensionsInfo.com provides a size guide covering the smallest to the largest hook size. The site also describes extra-strong hooks, hook variations and the hooks needed for specific species. Hook sizes are illustrated and numbered for easy comparison.