Do you by any chance have any monitor / video card software running?
Samsung monitors typically come with tools that can be installed alongside the driver to control the display features from Windows. A lot of the time this involves hijacking application windows and, well, there could be a bug in the software causing it to be resized.
Ditto for video card software like nvidia or ATI which typically have overlay / window management features baked into the drivers bundleware.
I would look at those as the culprits before mIRC.
I've seen mIRC forget its placement / size before when restarting the program, but I've never seen mIRC move/resize itself at some random point mid-execution. mIRC never resizes itself on its own, it doesn't have any need to do this, so the only other explanation would be a script. I assume you ruled that one out.
