Where can I enlarge the font used by the utility panel?
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It uses the system dialog font, so should be the same size as dialogs in other Windows desktop apps. (e.g. Right click the taskbar and choose Settings.)
You can increase font sizes (or the size of everything if you adjust DPI rather than just font size) through Windows settings, but it will increase font sizes everywhere. That may be what you want if the other dialogs (e.g. Taskbar Settings) are also too small.
Opus lets you adjust font faces and sizes for some of its UI elements independently via Preferences / Display / Colors & Fonts, but the system dialog font is used for most dialogs (including the utility panel, and Preferences itself), at least at present.