[ianvisits]

At a guess - I suspect they are assessing risk of investing time/money in installing your product into their company and how they would deal with issues if you are off ill one day or want a holiday.

You need to be honest about your company size, but turn it into a positive, such as sole developer, backed by a third-party support package (assuming you have one), or that the company is expected to grow fast, so they wont be relying on just one person to fix things etc.

Pitch it really well, and depending on the type of customer, they may even end up wanting to invest in your company to help you scale up faster than you could expect (yes, I have seen that happen with start-ups).

Don't be afraid of awkward questions like these, as they will crop up, and each time you gain experience in handling the RFP style enquiries, and with experience comes improved skills to close those sales deals.