What is the difference between commercial farm and small farm?

In general terms, they are defined not by the physical size of the land they occupy, but by the revenue generated. A small farm has revenues that might partially support one family, but would allow for little growth of the family or business. Commercial farms generally have enough income to support a large, multi-generational family and possibly the families of several employees as well.

For example, a 100 acre midwestern US crop farm raising corn, soybeans, and some hogs would likely not fully financially support even one small family. But a 100 acre greenhouse operation producing a variety of nursery plants and/or produce would be a comparatively huge operation, requiring a great deal of physical and financial resources, but generating revenues in the millions of dollars.