It seems to me that the first thing to do is to establish whether this is a general issue on 4K screens, either for my script or for dialogs in general (though if the latter I would have thought it would have been reported by others by now - unless it is the use of options notheme that is causing the issue).
So if anyone else with a 4K screen can check whether they get the same issue - or conversely confirm that they don't - then that would also be helpful to point to whether this is a mIRC issue or something specific to this user.
It may be impossible to replicate or resolve without direct access to his machine to test on.
We have yet to establish definitively whether this is specific to this user or a more general issue.
It may be possible that custom 'large icons' or 'font zoom' settings on some version of windows behaves unexpectedly and outside of the scope of DBU scale, blowing up the font point scale instead.
Anything is possible.
If you can give more precise instructions on what Windows settings should be checked that would be helpful.
Just write a custom scaler that doubles everything in your dialog. Add the option: "Jumbo Mode." Or half the point sizes of your font formattings.
I will not be writing a custom scaler that doubles everything in my dialog - that is really not the solution here. This is either common on 4K screens in which case it is a mIRC issue, or specific to this user's settings in which case he needs to fix his settings - either way it is not the script writer's responsibility to find a kludge workaround in order to resolve an issue for a single user.