When I switch the different responsive widths (XS, SM, ...) the preview should show the same settings. At the moment I need to resize the browser window to see the effect in the preview.
Are you talking about the preview in the app or in your web browser? Switching that setting doesn't do anything for the web browser, it can't because the browser is reading it according to the viewport size. What you "can" do though is use this site to see what your site looks like at the various viewport sizes: http://whatsmy.browsersize.com/
It will allow you to resize your browser manually until you get to the size you want. Not perfect but it works pretty slick and will give you a good idea of the looks of the pages at the specific sizes you're trying to work with.
I meant the preview in my web browser. How about setting the window dimension of the browser out of BSS? I mostly run my browser in full-width and alone this would save fiddling around with it.
Are you using Chrome? If so hit F12 for the Dev Tools then click the Toggle device tool (CTRL+SHIFT+M) this will give you a device tool bar in the browser's preview window that you can switch between viewports basically.
Saj