With all Selenium-based connectors simplest way to change default window size is to open Settings / Selenium menu and append a command line argument window-size=width,height, i.e.:
["window-size=600,400"]

If you have other options, then you may combine them using the comma, i.e.:

["disable-popup-blocking","window-size=600,400"]

Press Save to save the profile and the press Test to see that browser opens with desired dimentions.

Legacy Connectors

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.