We are always told that accomplishments on a resume must be quantifiable. How can "drove a sales increase of 20% to $50 million" be restated so as to not divulge financial data from a prior employer? Because simply saying "drove a 20% sales increase" is much less impactful.
Drove a 20% sales increase at a multi-million company
You can put in hard numbers and not be too explicit at the same time. If multi-million is still too much information, that makes it harder. But most people who are familiar with other companies in the area would have a general idea of the value of that company. Use similar language to how it would be described by someone who doesn't know the details.
Know what data is public and work from there. Companies on the stock market have to publish financial details. Review what's been published and see if there's a way you can connect with that information.
Also - you can imply, as @thursdaysgeek says - "Drove a 20% sales increase in a ...":
- company in the fortune 500
- multi million dollar company
- product line worth 10s of millions of dollars annually
- profit center worth relatively X amount of the companies annual profit
Be careful and claim what's real here. Hard numbers are impressive, inaccuracy is not. IF you really are the leader of the single initiative that drove a $40 million dollar profit to $50 million dollars - then go for it. But if you were one of several initiatives and it's hard to say which of you made all that money, be clear that you are "part of" the initiatives.
Also speak how the field speaks. In defense contracting, where certain elements of projects where often classified, I learned that if you say "X year, Y # of staff initative" you define fairly well both the size of your project, the complexity and the amount of money involved without having to say what project, or what budget. But the same conversion factor isn't true across industries, so that's not a clear metric when talking to someone out side of that world.