Find the install directory for Moviestorm (C:/Program Files/Moviestorm General Release, if you went with the default).
From that folder, open in a text editor (such as Notepad).
Edit the following line:
Using much more than 1024M will probably cause problems with Moviestorm, so it is a sensible maximum.
You forgot to mention the file that needs editing, the file is Moviestorm.l4j.ini which is located in the main directory, or at least thats the only file I could find in my quick search which had that line.