Can you put a smaller diamond in a 10 mm setting and what is the smallest?

A jeweler can help you refine the setting to accommodate a smaller stone -- 10 mm being about the size of a four-carat round stone. Stepping down a full carat in a round cut, means shrinking the setting down to 9.3 mm, for example.

The key will be to preserve the proportions of the setting, which was designed to show off a four-carat stone.