Just a random guess, but do you have apoc.export.file.enabled=true written in your conf?

Also, I'm pretty sure apoc.export.json does not support batch size (only cypher does).

Also, it seems odd to me that you specify "null"-are you trying to stream this data somewhere, or save it out to file? Because if you are saving out to JSON, maybe something like:
apoc.export.json.all("/your/path/here", {{useTypes:true});

I was able to recently use APOC to export a database with 1M nodes and 4M edges with no issue, so I don't believe it's a size issue.