Here's an example of how you do it:
Code:
map_data = Marshal.load(File.open('Data/Map002.rxdata'))
map_data.width = 10
map_data.height = 9
file = File.open('Data/Map002.rxdata', 'wb')
Marshal.dump(map_data, file)
file.close
p 'Reduced the size! Please reopen your project.'
Throw this script above Main and it will resize your second map to 10x9. The editor will still say 20x15, so be sure to close and reopen your project. Now it will say 10x9 (but don't try to change anything in the Map Properties as you will be prompted to change the map size). On the map editor, it will still appear as 20x15, but everything beyond 10x9 is ignored in-game. Simply remove the above script and see for yourself.
Granted, if you want to change ALL of your maps to this size, then I can whip up a loop for you so you don't have to manually run the above script for each file.