Trello – How much storage space in Trello

Hit up the following site. It's in the middle, under "Storage": https://docs.google.com/settings?hl=en

This is not the intended purpose of github. It's intended for management/perusal of software source-code (and related assets).

Please also see this article: https://help.github.com/articles/what-is-my-disk-quota

For standard online storage, I would look at Dropbox, SkyDrive, Google Drive, Amazon S3, or an online backup solution.