You can find the local cache of your Steam cloud files at ~/Library/Application Support/Steam/userdata/<your account id>/255710/remote
In that folder, you should find the save files you have previously done on the cloud, copy them to a safe place so you can always copy those back to your local save folders if you want to.
Then in the main menu of the game, open the Content Manager, in the Save games tab, you should see your cloud files (they will show a little cloud next to the delete button on the right). Click the delete button and it should remove the file from the cloud, hence freeing some memory. Try to remove a couple of them and attempt to save a game again, if it works, then it is a problem with disk space. (Also we are working on adding a proper out of space error message in the next patch)
Please note deleting or adding files directly in the Steam remote folder has no effect on the files actually located on the cloud, the only way to actually delete the save from the cloud is to do it from the Content Manager.
Let me know how this pans out !
Thanks