The maximum room size in editor is 256x192 (the z80 screen), though it can be forced to bigger values for non-DizzyAGE games.
If you set your room to this size, you must also adjust the view position in the game. That is where the room's view is displayed, relative to the 245x192 screen.
By default, in handlers.gs you have:
In your case you probably need to set it to 0,0 (top-left corner) and change the look of the hud.
The room size should be received through game variables G_ROOMW and G_ROOMH, which would be the values set in the editor.
There might be other things to configure, depending on scripts, but this is a start.
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