Viewport size for Media Query testing

It would be wonderful to be able to select a size or device size when viewing files on iPad so that true mobile testing can take place.  

Perhaps a few select sizes, such as mobile, tablet, and desktop to begin, or perhaps a width and height control for viewing how Media Queries are behaving?

Thank you for taking this into consideration....and I could never tire of thanking you for the wonderful app you have provided.  I really enjoy using it and feel very confident developing website solely on my iPad Pro.