This has already been brought up in the official OME thread, but people seem to be having trouble with it. This is just a workaround until Osici adds large map support (unless it's some more complicated issue).
These are batch (.bat) files for windows. Just place them in the same directory as OME.jar, and run them. They will run OME with higher memory, allowing it to load larger maps. Obviously, doing so uses more ram, so y'know, be wary of how much you have.
If anyone has better explanations of how this uses ram, feel free to correct me/elaborate. I'm not quite an expert on it, I just copied the commands from the official thread, and made batch files for them.