How do I change the font size for the channel tabs? tried changing at view -> fonts but it doesn't affect the font for channel tabs. Also tried changing at windows' display option but to no avail.

Click on mirc icon, on channels titlebar, and click "Font". Then set as default.

nope, doesn't work

If you're talking about that, you can't change it.

You can make it change to bold when it's the active window by going
to options (alt+o) > display > options > 'Visual styles' button

de-select 'Light switchbar font'