I added this secret configuration a couple of weeks ago. There will be a proper “fullscreen” option added with one of the next updates. Until then, editing the config file is the way to get fullscreen.
In the newest update, fullscreen is now possible without editing configuration files.
Simply press ALT + ENTER in the FWsim main window. This will make the show editor, the effect editor and the world editor fullscreen.
FWsim Basic and FWsim Creative Edition are now full-screen by default. FWsim Pro is shown in windowed mode by default but can be switched into fullscreen.