No kidding, I've done 20.8MB and the heap size of our JText is set to 1200MB (I misstated in my previous post), so we could go higher. Granted that was 20MB of xml so there is a lot more "filler" in ...
That error is coming from the JText agent process, not your MQ system. How big of a file are you trying to read/write? The biggest I've been able to handle is a 20MB xml file with a 2GB heap.