If you have a document in writing indicating that the employer would pay severance upon termination, the terms of the document should be legally enforceable as long as it was signed by an individual with sufficient authority to bind the employer.
However, the specific issue as to whether or not severance would be paid in your situation will be controlled by the language of the contract as applied to the circumstances of your termination. in other words, the plain language of the contract will be controlling.
While the number of employees may matter in some areas of employment law, this will not be relevant in your situation. I recommend having your document reviewed by an attorney to get an opinion as to whether or not you are entitled to severance based on your circumstances. From there, the lawyer can advise you as to both informal and formal ways to pursue the severance.
Absent a contractual reason to pay severance, one is not owed. If you have an agreement that you believe is enforceable, it would have to be reviewed to determine the viability of any claim. Hope this helps.
Absolutely contact an employee benefits attorney in your area. If it's an enforceable contract, then legally you should get your money. Moreover, severance can be covered under state wage payment laws.