I think you want to know the size of the Website Width in pixels. If so, it seems to me that Storefront is 1950px. If my Header is 1950 and Background 1950 and centered, then they match up. Any more and Storefront starts to do variable cropping/resizing.
The height of the Header (top to bottom) is variable. The suggested size is 500px but that changes as soon as you remove the Title and Tagline, then it SHRINKS to about 175px. Increase the size font of the Title and/or Tagline (using Plugins) and you can get a larger Header, only it might cause trouble with Menus.
It seems RIDICULOUS but I’m sure it is Web-Designer magic. The variable stuff makes everything (unaltered) look good in any Browser, Screen, Device etc. I’m still looking for a Pay-For Plugin/Extension that gives me more choices in design/layout.