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.
nobody is using it CURRENTLY I meant. Also almost nobody has any coins to pay for storage with. There is a lot of infrastructure still growing and needed before we can worry about how many TB/1mbit line like the OP was asking.
I'd lower my storage fees to almost zero but still can't get my portforwarding to announce. ports are wide open in netgear..