I just checked the source. There are no hard font sizes set in Sigil source for any dialog or even the main app. All Sigil font sizes are inherited from your OS system or from Qt itself. Please go through your OS appearance (Control Panel) settings and make sure the minimum font size is set according to your needs.
I am not a Windows user so I can't help much but I did a quick Google search and found this link which leads you hrough changing minimum font sizes for your OS.
http://www.windowscentral.com/how-ma...ger-windows-10
Not sure if that is the best link or not but hopefully something there will help.