I just tested with up to 4MB of data in the upload of a post event, which should be more than enough for a vast majority of use cases, when you communicate with your server.
Still I wonder, what the maximum is, that GameMaker supports on its end?
One of the rare use cases, where 4MB might not be enough is kind of a cloud sync for complex save games, avatar upload and other image-heavy data.
Anyone knows, how much i can transfer?