Make pixel size numbers in Preview show larger when editing photos?
It's not possible to customize the size of the said on-screen numbers shown by Preview.app.

As a workaround and a more generalized solution for other such cases, you can leverage the Screen Zoom feature available in macOS. Although it's an accessibility feature, it is generally employed by UI designers to create pixel perfect UI.

To enable and configure Screen zoom, go to System Preferences app → Accessibility → Zoom, and enable the checkmark for Use scroll gesture with modifier keys to zoom. Now assign a convenient key to be used as modifier key (I use Control).



Once configured, you can press and hold the modifier key and scroll to quickly zoom in and out of your screen.