[Challenge] If you could right-click the sky...

[BAndrew]

I don't think that what you ask can be done, unless you can use a ruler (for example) to compare your monitor's size with the image's size.

so if a is picture's size and b is your monitor's size (which you know it):
and for instance
a/b = 1/4 =?
a= 1/4 * b
you know b, so bingo