If you are on Windows 10 1703 or later (or possibly 1709 or later) then there is a compatibility setting:
This is definitely a work-around for a genuine issue of display scaling support in mIRC, and of course it only works if you are on Windows 10 1703 or later (so tough if you are still on Windows Vista or 8 or 8.1 or 10 RTM or 10 1607).
Using the compatibility settings in the image above (or the All Users version), select the "Override high DPI scaling behaviour." checkbox and select "System (Enhanced)". (System also works but does it by emulating a lower resolution and fonts are blurred.)
@Khaled - when you are trying to recreate the issue, you might want to check that you haven't got this compatibility option set - or if you are on 1803 that you haven't got Windows 10 1803 automated version of this turned on.