I assume you're on the "Enter your purchase information" page.
That number of shares box is not formatted for dollars it's formatted for shares but it's limited in size; I think 99,999.999 is the limit.
You'll have to break that 2 million share exercise into smaller pieces if you want to enter that 2 million share amount.
You could go the other way, of course and divide the 2 million shares by 100 and multiply all the "per share" figures by 100, but when you go to sell, assuming you're using TurboTax, you'd have to handle the sale in a similar fashion.
Tom Young