Only if mIRC is maximized (or use a transparent @window that pops up just for calculating)

$window(-1).h returns the screen height

lets start calculating wink

mIRC is maximized and $window(-2).h returns mIRCs height
$calc($window(-1).h - $window(-2).h) = your taskbar size blush

if your mIRC isn't maximized use something like this:
Code:
alias calctaskbarsize {
  if ($appstate == maximized) { return $calc($window(-1).h - $window(-2).h) }
  else {
    window -dnh @calctaskbarsize
    if ($istok(2K3 2K XP,$os,32)) { setlayer 0 @calctaskbarsize }
    window -x @calctaskbarsize
    var %s = $calc($window(-1).h - $window(@calctaskbarsize).h) 
    window -c @calctaskbarsize
    return %s
  }
}

one step closer to world domination