Hmmn. I wonder how it handles running out of space? Or, come to that, the need to export one notebook at a time (because there's no metadata for notebook and restoring a mixed export would put all the notes in one notebook). You might want to ask the developer about that.
As to database size, the storage used on your desktop device will differ from the exported size, because the github app converts your content into (I think) a SQLite database and I have no idea whether that is more or less efficient than Evernote's native storage.
To get a general idea you might search for any Evernote folders on your device and check their total size - on a Windows device the content will be here (unless you changed it) - C:\Users\<you>\AppData\Roaming\Evernote
I have around 68,000 notes and my overall storage at that location is 38GB - as a very rough estimate your export should be around 20GB (but you may need somewhat more space while the app is processing the export.)