Perhaps I'm missing something, but I see no way to adjust the type size in either the preview or markdown panes. Using Joplin on a 15" MBP the default typesizes are way too small to read comfortably.
So consider this a feature request. Also a way to select which fonts are used would be nice. Thanks.
There's a "Set application zoom percentage" on the options that will apply to the viewer and editor.
I'm not sure if the "Global zoom percentage is really working. It seems like it's not doing anything. Here's the editor in 100% (default) (Windows 10):

And this in 300:

Just for a test, this is 1500:

I don't see any difference :/
I can confirm that this is not working on Linux (Debian).
It behaves in the way described by @Minh049 - so it does not work at all. :-(
/cc @laurent22
Also confirming here not working on MacOSX, version 1.0.104.
this works with Linux Debian 9
After changing the value, I did "apply" then "ok" then I restarted the app.
Didn't work for me in Windows 10,
but after following foxmask steps (click "apply", restarting app at the end), it worked!
Is it possible to change font size without changing zoom level?
@jaPyR did you try:
<style>
body { font-size: 2em; }
</style>
Custom CSS is also supported via a stylesheet - https://joplin.cozic.net/#custom-css
Most helpful comment
this works with Linux Debian 9
After changing the value, I did "apply" then "ok" then I restarted the app.