I've updated this. New features:
Smarter memory management, allowing maps to be generated and saved at higher resolutions.
New special commands: "Clear Map Cache" disposes cached bitmaps so that new ones can be generated at the highest resolution available; "Generate and Save All" disposes bitmaps after they are saved so that each map is saved at the highest resolution.
With the new updates, a 500x500 map can be saved from 2x zoom, whereas it might have been at 4x or 8x before. Please note that saving the PNG file takes a very, VERY long time at these higher resolutions. A 4000x4000 image (the size of a 2x zoom map of 500x500 tile size) is likely to take a long time, and do remember that the RGSS Player window MUST remain in focus during the whole process; focusing on another window will pause it. I can not think of any way around this, since it is built into Enterbrain's design.
And don't be surprised if your 150x150 map takes more time than your 500x500 to save; a 150x150 map, at full scale, is 4800x4800 pixels, while a 500x500 map at 2x scale is only 4000x4000 pixels, so the smaller map will probably take less time to generate, but more time to save.