Try just adding another 10 like so...

 Code:
Function PixelStrSize($sInput)
  Dim $, $i, $c
  $ = SetOption("CaseSensitivity", "On")
  For $i = 1 to Len($sInput)
    $c = SubStr($sInput, $i, 1)
    Select
      Case $c = "'"
        $PixelStrSize = CDbl($PixelStrSize) + 2
      Case InStr('fjt !,.:;-/\()"Jr', $c)
        $PixelStrSize = CDbl($PixelStrSize) + 3
      Case InStr("szkvxy?cL", $c)
        $PixelStrSize = CDbl($PixelStrSize) + 5
      Case InStr("ughdeK0123456789$BEFYZabnopqP", $c)
        $PixelStrSize = CDbl($PixelStrSize) + 6
      Case InStr("UVRSTNACDGH*+X<=>&#OQ", $c)
        $PixelStrSize = CDbl($PixelStrSize) + 8
      Case InStr("Mwm", $c)
        $PixelStrSize = CDbl($PixelStrSize) + 9
      Case InStr("@%W", $c)
        $PixelStrSize = CDbl($PixelStrSize) + 12
      Case 1
        $PixelStrSize = CDbl($PixelStrSize) + 10
    EndSelect
  Next
  $PixelStrSize = $PixelStrSize + 10
  $ = SetOption("CaseSensitivity", $)
EndFunction