Does TN Visa have requirement for company?

There are a lot of types of companies out there:
Sole Proprietorship,General Partnership,Limited Partnership,C Corporation,S Corporation,Limited Liability Company(LLC)

Every type of company can hire employees. So they can ALL issue offer letter to Canadians to apply for TN visa? Is there any limitation for the size of the company? Some types are subject to more strict scrutiny at the border like small company?