I don't know if Dropbox has a limit on file size, but whether or not they can do that, it's not their positioning. They're all about "sharing files", but to say "we let you share big files!" would be too narrow, and turn off or confuse more people than it would attract.

Perhaps you should be recommending yousendit.com or similar

I do recommend yousendit instead. For me, the "sending big files to someone" is a great hook to get them using the rest of dropbox' features.

Judging by the downvotes, I guess nobody agrees with me :)