resolution - How to size images independent of device?

I’m a writer and I just bought a Microsoft Surface. I often have to size images with exact dimensions to fit themes. For example, I write for a site where the images have to be exactly 1345 pixels wide. So far, I used do this:

1. Take screenshot (print screen key)


2. Open MS Paint


3. Crop/Resize image so that the pixel count on the bottom matches what I want.

Now with my new Microsoft Surface, I’m facing a weird problem. 1345 pixels on my new screen is much smaller than what I’m used to! So if I take a screenshot on Surface, paste it into MS Paint, and crop it to 1345 pixels, the portion of the screen it captures is ridiculously low.

What do I do now? I thought pixels were fixed width. So now what measure do I use to ensure that my images are the same “size” regardless of what computer I’m using? How do I replicate 1345 pixels on my Surface to capture the same screen area that it used to on my old laptop on MS Paint?

If I need to use another software, I’d prefer to use something simple without a gazillion options, since the only thing I really need to do is crop/resize.