For private companies, I suspect you may have an issue dealing with pricing your product based on their revenue.<p>If your product is something that multiple employees use, then it is easier to make the definition based on the number of people.<p>You didn't really give us much to go on. 
Take a look at how companies like 37Signals (basecamphq.com), or freshbooks.com seperate their offerings.<p>Hopefully for you, there is a model which is either # of people using your service, or some other metric you can charge by.<p>For instance, if only 3 people in a small company would use your product, but only 4 people in a large company would use it, then you can't really price based on # of people in the company (or else, how would you know when the company has grown to the new pricing).