The built in rounded corner layouts are not super flexible since they use images to round the corners. It uses images so that the layouts are 100% compatible with Internet Explorer. So, adjusting the width like you said will not have the desired effect until the images are also updated to be wider.
I made some custom CSS Sculptor presets that use CSS3 to round the corners and doesn't rely on images at all. The upside is the designs are very flexible and updating the width will adjust itself correctly. The downside is Internet Explorer won't round the corners at all and will just use square boxes instead. These designs are unsupported by WebAssist, but are still cool to check out. css-sculptor-css-rounded-corner-presets/