How to increase Text size of the system?

I've never use an external monitor but have never seen an option to scale the system text. With Mojave one can change the scale of the entire screen which, in turn, changes the size of the text in menus and tool bars. Here's what I use:

However, I don't know if this carries over to an external monitor.

Changing text size has never been available except within certain apps such as Pages. As Old Toad mentioned, you can lower the resolution, which will increase the individual pixel size resulting in everything appearing larger, but will also result in a less than crisp and clear picture including text. And it does carry over to an external monitor as long as that resolution is supported.