Typical patio umbrellas are 9 feet in diameter. Anything smaller than that can be considered to be a small market umbrella. 6 foot is a typical size for smaller umbrellas, such as the following items.

4.5, 6.5, 7, 7.5, and 8 foot models can also be found. Perhaps surprisingly, 9 foot models are sometimes cheaper than the smaller models, probably because the larger umbrellas are more common.

Hope this helps you in your search for a small market umbrella.