Data models greater than around 10 megs are not currently supported. It sounds like you might be running into this limit.
You should think about how you can reduce the amount of data you are storing. E.g., store large items like images outside of the Realtime model, or (based on what you said you were doing in your previous question) do some smoothing to reduce the number of points stored as it increases.
You can export the data model right now in the Javascript API: https://developers.google.com/drive/realtime/reference/gapi.drive.realtime.Document#gapi.drive.realtime.Document.prototype.exportDocument
But in order to do that, you have to load the document first.