OK, there is no direct measurement between pixels and inches for the screen - it all depends on the physical screen size, not just the number of pixels.

Typically people talk about 72pixels per inch and 96pixels per inch, but it's not a guaranteed thing.

Two laptops may have a resolution of 1280x800. One may be a 13" laptop and another may be a 15" or even a 17" laptop (or screen). So they may share the same resolution but have differrent physical sizes.

You can't reliably predict that for a given pixel size it will be a particular physical size when the viewer sees it.

Is that as clear as mud?

Is there any particular reason you need accurate screen sizes for your pixel images?

Perhaps a good example of this phenomenom is the humble TV set. You can buy a HD TV set in all kinds of sizes, but they often share the same pixel resolution.