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Do you want to do this only for yourself? In this case, it would be enough to increase the fontsize within your browser e.g. using the shortcut cmd + on MacOS or ctrl + on Windows. Also reducing the size of your browser window would work in that direction.
Afaik, there is no way to simulate this behaviour easily with simple adjustments in CSS. If you want this change for all visitors of your forum, you'd have to change a lot of CSS rules (and likely more) regarding the reponsive design, as @clarkwinkelmann already pointed out.
The <meta name="viewport" ...> declaration was introduced to handle, how mobile devices should scale the content of websites to adapt to the reduced screen estate, but I have never tried whether this can be used on desktop computers to emulate a mobile device.