Simple. Describe your company by the gross revenue you have accumulated to date. All. Everything.

There are actually only two ways to describe the size of a company: headcount and revenue.

I agree that your headcount won't likely give the impression of a large company. So you need to go by revenue.

For the sake of earning respect from your prospects, to make the number seems impressive, you can add up all revenue you have generated from day one you started the business, yes, including the revenue when you probably is still doing freelance work without a team. Add everything up. Then use sentences like "As a business we have generated $XX,XXX,XXX of revenue to date.", which is a fact. Don't bother to mention how many years you need to hit that level or revenue. You can also add "Our team is very, very efficient", which your prospect will also see as true, as with 10 people only you are able to generate such amount of revenue. You can also add those big names. Then you highlight all benefits they get by working with you.

By now I am impressed by your business already! :D

Give it a try. Let me know how it works. Good luck with your business and don't forget that I am always open for call for further discussions.