UziMonkey is right. If you want more information we need to know specifically what you're trying to change. Are you only interested in changing the text of items in the taskbar? That seems odd since it is usually just the titles of minimized or active programs.
What about active programs that you are using? Do you want the font and size changed on those as well?
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If it's anywhere, it's in Control Panel -> Display -> Appearance -> Advanced. If it's not there, a registry hack or third party program might be required.