How do I resize the vertical and horizontal markers for the Paint page? I know they should be located in the middle of the vertical or horizontal edge but they are so small that I struggle to find them.
In below screenshot I've circled the markers I'm referring to:
You cannot resize those markers only.
Scaling up the whole Windows interface does increase those markers a bit, but it also scales up the rest of your interface.
If you find it hard to click those markers you can also resize the page via File ➡ Properties (or press CTRL+E):
