Eastern and Gastec are retailers that provide propane tank size charts on their websites. Additionally, Propane 101 is an online resource that gives information regarding propane tank sizes for commercial and residential needs.
Eastern offers propane tanks for sale that can hold a capacity of 57 gallons, 120 gallons, 500 gallons and 1000 gallons; a picture including the width and height of each tank are also listed. Gastec provides similar charts of its tanks, and uses a human model to show comparison of size.
Propane 101 website has more in-depth information on propane tanks. The resource gives insight to tank sizes, their uses, how to determine capacity limits and installation tips.