Google-drive – Check how much storage space I have left on Google Docs

Trello FAQ says:

There is a 10mb file upload limit per attachment. It's not possible to upload attachments via the mobile site. There is no limit on the number of attachments you can have on a card or board. Neither is there an account data storage limit.

Just announced:

Bringing it all together: 15 GB now shared between Drive, Gmail, and Google+ Photos

With this new combined storage space, you won’t have to worry about how much you’re storing and where. For example, maybe you’re a heavy Gmail user but light on photos, or perhaps you were bumping up against your Drive storage limit but were only using 2 GB in Gmail. Now it doesn’t matter, because you can use your storage the way you want.

So, it looks like you only need to wait until they roll this out to you, which will probably be within the next few weeks.