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.
First of all, why do they know or want to know your size? I'm not saying to lie: I'm saying that if your website says: "We are a 10-person company ...." then you're not leading with your best differentiator. Don't volunteer information many won't ever ask for. If they ask, tell the truth. But what's the truth? If you know they're going to ask, because it's a required DoD question, e.g., then immunize yourself by addressing it first. If you have 11 people, you also have fewer than 25 but more than 10. Never make yourself smaller than you have to be, as it's already obvious you're not a behemoth or they'd have heard of you already. That's why I'm less than 6 feet tall, rather than "taller than 5 feet."
But beyond that, lead with your big company clients. If you tell me that IBM, P&G, Cisco and Merck are your customers, and you've got testimonials from each of them, then that's all you need. Lead with it. They are your complete validation. Now, if they ask about your having 10 people or you need to tell them that, your answer is simple: "That's exactly the question IBM, P&G, Cisco and Merck asked and in fact, they hired us because at only 10 people, we can be <explain what you do better than anyone else>. They've been so happy with us that they've doubled their business with us in the last 3 months. In fact, we expect our company to double in size in the next 12 months (again, assuming that's your actual projection -- use truth to buttress your case)." I've beaten much larger companies out -- for over 30 years -- using precisely these tactics.
Good luck to you. Happy to discuss should you have any questions.