I suppose it is the progress bar that he finds too small.
One solution is to create a custom GUI progress bar with the desired size, but why reinvent the wheel. Just adjust the size of each component :
#include <WinAPISysWin.au3>
#include <String.au3>
#include <WinAPIGdi.au3>
#include <FontConstants.au3>
ProgressOn("Test", "Main text", "Sub Text")
Local $hWnd = WinGetHandle("Test")
WinMove($hWnd, "", 200, 200, 600, 250)
Local $hText1 = ControlGetHandle($hWnd, "", "Static1")
Local $hProgress = ControlGetHandle($hWnd, "", "msctls_progress321")
Local $hText2 = ControlGetHandle($hWnd, "", "Static2")
Local $aPos = ControlGetPos($hWnd, "", $hText1)
_WinAPI_MoveWindow($hText1, $aPos[0], $aPos[1], $aPos[2] * 2, $aPos[3] * 2)
Local $hFont1 = _WinAPI_CreateFont(50, 0, 0, 0, 400, False, False, False, $DEFAULT_CHARSET, _
$OUT_DEFAULT_PRECIS, $CLIP_DEFAULT_PRECIS, $DEFAULT_QUALITY, 0, 'Arial')
_WinAPI_SetFont($hText1, $hFont1)
$aPos = ControlGetPos($hWnd, "", $hProgress)
_WinAPI_MoveWindow($hProgress, $aPos[0], $aPos[1] + 50, $aPos[2] * 2, $aPos[3] * 2)
$aPos = ControlGetPos($hWnd, "", $hText2)
_WinAPI_MoveWindow($hText2, $aPos[0], $aPos[1] + 85, $aPos[2] * 2, $aPos[3] * 2)
Local $hFont2 = _WinAPI_CreateFont(40, 0, 0, 0, 400, False, False, False, $DEFAULT_CHARSET, _
$OUT_DEFAULT_PRECIS, $CLIP_DEFAULT_PRECIS, $DEFAULT_QUALITY, 0, 'Arial')
_WinAPI_SetFont($hText2, $hFont2)
For $i = 10 To 100 Step 10
Sleep(500)
ProgressSet($i, $i & "%")
Next
ProgressSet(100, "Done", "Complete")
Sleep(2000)
_WinAPI_DeleteObject($hFont1)
_WinAPI_DeleteObject($hFont2)
ProgressOff()
If it is the MsgBox that is too small, you can pretty much do the same, but you will need to hook the MsgBox to a CBT proc.
In any case, I felt it is a good example of how to reuse an existing function without creating it from scratch...