It look like the width and height isn't taken when specified. I tried several time with different value, using different url or html content, played with css, but I always get a static window size with scroll bars. Anyone else have this behavior ?
I've just realize that it is doing this only when I check the option: Center on monitor with mouse.
Using a javascript to center the widow for now: window.moveTo(screen.width/2-400, screen.height/2-300);
ah interesting, I'll look into that!