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.