Seeing as no one is using the storage service, the best advice would be a guess. I'd say stick whatever you have in there. Don't spend an extra dime. If you fill it up, add more.
Nobody is using it because I set the default price at some absurd value, and nobody has lowered it. The next release is going to have a lower default, and I'm going to encourage everyone to lower their price until they start getting files.
Nobody wants to dump 60 KS on a 100mb upload.