Site means a physical location where some activity takes place and it should not be confused with a web site, which is virtual.
A site has an unique address and a number of other attributes yet to be determined.
Examples of sites:
Farm, Factory, Store, Shop, Restaurant, Office, Supermarket, Warehouse, Port, Airport, Cargo Terminal.
Basic distinction between small and other businesses can be roughly done by asking: "does the company operate in a single site or multiple sites?"