Steam workshop hit by zero MB bug - reuploading only solution
Since a couple of hours ago the Steam workshop has been giving problems - it manifests itself as showing a zero file size and subscribing won't do anything. A related problem is that you may not be able to remove the mod if you are already subscribed other then manually removing it.
If you have a mod like that contact the author and make him aware of the solution underneath:
Simply modify your upload to change the byte size and then push it to Steam again as an update.