Sorry, was on leave...

Nice function, does exactly what I need, but there's always a but... When I call the function, it seems to truncates at least the last char of the text in the text box. See my small example and try it yourself:

 Code:
;region ScriptForm Designer

;region Constructor

Break On
$System = CreateObject("KiXtart.System")

;endregion

;region Post-Constructor Custom Code
Dim $Ret
$Ret = SetOption("Explicit", "on")
Dim $
;endregion

;region Form Creation
;Warning: It is recommended that changes inside this region be handled using the ScriptForm Designer.
;When working with the ScriptForm designer this region and any changes within may be overwritten.

;~~< Form1 >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Global $Form1
$Form1 = $System.Form()
$Form1.Text = "Just a simple test"
;~~< TextBox1 >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Global $TextBox1
$TextBox1 = $Form1.Controls.TextBox()
$TextBox1.Size = 183, 20
$TextBox1.Location = 89, 93
;~~< Label1 >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Global $Label1
$Label1 = $Form1.Controls.Label()
$Label1.Text = "My Box:"
$Label1.Size = 70, 23
$Label1.Location = 13, 96
;~~< Button1 >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Global $Button1
$Button1 = $Form1.Controls.Button()
$Button1.Text = "Resize"
$Button1.Size = 75, 23
$Button1.Location = 61, 184
$Button1.OnClick = "Resize( $$Button1 )"
;~~< Label2 >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Global $Label2
$Label2 = $Form1.Controls.Label()
$Label2.Text = "Enter some text and press "+Chr(34)+"Resize"+Chr(34)+" to resize the width of the textbox"
$Label2.Size = 259, 46
$Label2.Location = 13, 24

;endregion

;region Custom Code
$TextBox1.text = "123.25GB"
;endregion

;region Event Loop

$Form1.Show
While $Form1.Visible
	$=Execute($System.Application.DoEvents)
Loop

;endregion

;endregion

;region Event Handlers

Function Resize( $object )
  $TextBox1.width = PixelStrSize($TextBox1.text)
EndFunction

;endregion
;region Script Settings
;<ScriptSettings xmlns="http://tempuri.org/ScriptSettings.xsd">
;  <ScriptPackager>
;    <process>kix32.exe</process>
;    <arguments />
;    <extractdir>%TEMP%</extractdir>
;    <files />
;    <usedefaulticon>true</usedefaulticon>
;    <showinsystray>false</showinsystray>
;    <altcreds>false</altcreds>
;    <efs>true</efs>
;    <ntfs>true</ntfs>
;    <local>false</local>
;    <abortonfail>true</abortonfail>
;    <product />
;    <version>1.0.0.1</version>
;    <versionstring />
;    <comments />
;    <company />
;    <includeinterpreter>false</includeinterpreter>
;    <forcecomregistration>false</forcecomregistration>
;    <consolemode>false</consolemode>
;    <EnableChangelog>false</EnableChangelog>
;    <AutoBackup>false</AutoBackup>
;    <snapinforce>false</snapinforce>
;    <snapinshowprogress>false</snapinshowprogress>
;    <snapinautoadd>2</snapinautoadd>
;    <snapinpermanentpath />
;    <cpumode>1</cpumode>
;    <hidepsconsole>false</hidepsconsole>
;  </ScriptPackager>
;</ScriptSettings>
;endregion

Break on
$ret = SetOption("explicit", "on")
Function PixelStrSize($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
  $ = SetOption("CaseSensitivity", $)
EndFunction