Many large companies worry about investing in a product or service provided by a small company. They fear you don't have the staying power. We are only 10 employees but we have many customer success stories including some very large companies. Looking for unique ideas on how to answer this question in a way that plays more to our benefit.
Focus on why they are asking. Ask them specifically what their concerns are - financial stability, response time, availability of your key people for their business, etc. Once you understand the why you can talk to how your company can address it - they really don't care how many people are there - there is another reason they are asking.