Real time resizing is clearly not a good method. Slow prehistoric scripting.
There are no full frame rate with a window > 800*600 unless you force a timer. (crap) So forget every neat animation in those large resolutions.
You can set the resolution to 320*200 under XP (if it's supported by your card and monitor) but not since the panel control.
You have no ideas about picwin scripting. If mIRC isn't particularly graphics intensive, some of us likes to proves the contrary since years.
The scripts I talk aren't unique or old one. Changing the resolution is a basic. Too bad, mirc doesn't offer this option.