There are different ways to determine the "size" of a business, of course.
Eg we can look at total assets (the value of everything it owns), or total revenue (the total amount of money it receives from customers eg in a single year), or market capitalization (total value of all its publicly traded shares). Or in more layman terms, we can consider the number of employees it has, or the number of countries it operates in.
The point is that there is no particular point right before which we say, "Tjis is a small business" and right after which we say, "This is a big corporation". It's just a sliding scale.
It's analogous to trying to define "rich man", "average Joe" and "poor man".