I have always had Macs of one form or another. All have the ubiquitous Retina display. In earlier Macs, you could easily change the resolution to whatever was required, but the Retina displays are somewhat bewildering when it comes to their resolution. A 13.3″ display has a resolution of 2560×1600 pixels at 227ppi, but it isn’t actually possible to obtain that sort of resolution using the system control on the Mac, i.e. there is no 1:1 correspondence between image pixel and screen pixel. The best they can do is “Scaled” resolution which provides one of four options:
1024 × 640 = ×2.5
1280 × 800 = ×2
1440 × 900 (default) ×1.77
1680 × 1050 = ×1.52
Why? Because the Retina display uses pixel-scaling, so the display at the setting of 1280 × 800 is scaled at 2 times the actual resolution, giving a “high-resolution” of 2560 × 1600 pixels. So every pixel is doubled to four times the detail. So a 280 × 280 pixel image displayed using the default setting is scaled 1.77 times, which means it displays as 496 pixels, which at 227ppi, means it is 2.1875 odd inches on the screen.
How to fix it? Use one of the 3rd party utilities like EasyRes. It installs in the top menu, and you can easily convert between screen settings… although the negative is that at 2560×1600 pixels, things other than images appear *very* small.
A pixel is an abstract, size-less thing. A pixels size is relative to the resolution of the physical device on which it is being viewed. The photosites on a camera sensor do have a set dimension, but once an image is acquired, and the signal are digitized, image pixels are size-less.
For example, let’s consider TVs, and in particular 4K Ultra HD TVs. A 43″ version of this TV might have a resolution of 3840×2160 pixels (w×h). The 75″ version of this TV has *exactly* the same number of pixels – about 8 million of them. What changes is the pixel size, but then so does the distance you should view the TV from. The iPhone 11 in comparison has a screen size of 1792×828. For example, the 43″ 4K TV has dimensions of roughly 37″×20.8″, which means that the size of a pixel is 0.24mm. A 75″ 4K TV would have a pixel size of 0.41mm. An Apple Macbook Air with a 13.3″ screen (2560×1600 pixels) has a pixel size of 0.11mm.
As an example consider the image below. Two sizes of pixels are shown, to represent different resolutions on two different physical devices. The content of the pixel doesn’t change, it just adapts to fill the physical pixels on the device.
Likely more important than the size of pixels is how many of them there are, so a better measure is PPI, or pixels-per-inch. The iPhone 11 has 326ppi, a typical 43″ TV has 102ppi, and the 75″ TV has 59ppi.