You can force Rapise to launch the browser at a specific size by creating or changing the built-in web browser profiles to include additional command line parameters. They are different for each of the browsers:

Chrome HTML

--window-size=200,500

Firefox HTML

-width 200 -height 500

Internet Explorer HTML

Unfortunately, IE does not support setting the width/height. However since there are not any mobile browsers based on IE, it is less important than the others.