How can I tell how much disk space might would be needed for an auto backup?

My Discourse is set up to automatically back up to Amazon S3. However, I have learned (the hard way) that it needs a lot of disk space on the local machine to prepare the backup. It looks like the software prepares the backup on local server, then moves it to S3 and gets rid of the local work files. So there’s a moment there where disk use spikes and then the backup is shuffled off to Amazon and the local work files are deleted.

How can I know how much workspace the backup process will need? It doesn’t have to be exact, but right now, I am completely flying blind and this has already completely crashed my forums and taken them offline once before.