OK then, try this

First: Go to mIRC, size and position it as you want it. (Just the main mIRC window, not any of it's child windows)
Now click on the mIRC icon in the titlebar. In the menu that appears, select 'Position' > 'Save'


Next, copy the code below and paste it into the mIRC Scripts Editor (Press Alt+rr to open the editor) and press OK to save it.
Code:
on *:appactive:{
  if ($appactive) && ($window(-2).w !> 800) {
    sendkeys % {DOWN}{DOWN}{RIGHT}{ENTER}
  }
}
alias sendkeys var %a = $ticks | .comopen %a WScript.Shell | if (!$comerr) { .comclose %a $com(%a,SendKeys,3,bstr,$1-) }


Whenever mIRC is activated it will check that its width is not 800px or less and if it is, will reset itself to your saved size and position.
Note that I am assuming that Windows resizes it to 800x600. If that is not the case you can adjust the 800 in the script to suit. You can find the actual width of mIRC after resizing by typing //echo -a $window(-2).w (before you resize again obviously).