Thanks for your post.
I have made a change to the next version that should hopefully resolve this.
The issue you are seeing is due to the way Windows 32/64-bit application resources work, where the font/size is a fixed part of the resource definition. mIRC still supports Windows XP, which means it has had to use a dialog font that is usable across many versions of Windows.
I recently had to make changes to mIRC's resource handling to support language translation and, because of that, this has made it far easier to update a dialog template's font/size on-the-fly. I have made a change so that on newer versions of Windows, mIRC dialogs will now use the default Windows GUI font. On Windows 10/11, this is Segoi UI, whose size can be changed in the Windows Settings Accessibility Text Size setting.
The next beta release will include this change, so you can test it out to see if it helps.