mIRC only supports changing fonts in Status, Channel, Chat, and other MDI windows. I'm afraid that there is currently no way to change the fonts in dialogs, which use the standard Windows dialog font size.
If you find that the font size that Windows uses in its standard interface/dialogs is too small, one option might be to change your Windows display scaling. In Windows 11, this can be found in the Windows Settings / System / Display / Scaling dialog.