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The vast majority of shipping containers come in 3-4 different lengths. For international shipping, 20-foot and 40-foot containers are typically used because it's easy to stack them in a modular fashion. The term "TEU" is used to measure the capacity of a container ship. This works well with the two standard sizes of international shipping containers: one forty-foot container is the same "size" as two twenty-foot containers.

Here in the U.S., containers that are transported only on trains and by truck are usually 53 feet long. This coincides with the maximum length of a trailer for a tractor-trailer combination on almost the entire U.S. interstate highway system.

You may also find some 48-foot containers out there. China tried using them in international shipping for a while, but it was just too unwieldy to stack them with 20-foot and 40-foot containers. So they are generally used as smaller versions of the 53-foot containers you see here in domestic U.S. shipping.