The easiest way to customize the size of the text displayed is to adjust the zoom-settings of your browser. You can do this through the browser's menu, for most browsers, the corresponding shortcuts are Ctrl/Cmd + + (the plus sign) to increase font size and Ctrl/Cmd + - (the minus sign) to decrease font size. You can return to the standard zoom level by pressing Ctrl/Cmd + 0 (zero).
For high resolution displays like 4K monitors or TVs you can also change the scaling for the whole display through the computer's system settings interface.
On Windows, this is found in the Settings app.
Just select "System", then "Display" and adjust the value for "Scale and layout" as shown here:
On macOS, navigate to the Settings panel and click "Display", then select "scaled" and adjust to value to your liking by clicking on the small previews below: