title_body: browser tools to test custom viewport sizes?
upvoted_answer: Once you're in the responsive design view, the handles on the right side and bottom right corner of the view let you drag to set a display size. The drop-down menu in the upper left corner shows the dimensions as you drag. Once it's at the size you want, you can click on the menu, select Add Preset at the end of the list, and save those dimensions with a name.
  
  Here's a question that lists some resources for viewport dimensions: https://stackoverflow.com/questions/5784485/how-to-figure-out-common-viewport-dimensions-not-screen-sizes-of-common-device