To test this code, cut and paste the code below in a new standard project.

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Option Explicit

Private Declare Function SetWindowPos Lib "user32" _
(ByVal hwnd As Long, ByVal hWndInsertAfter As Long, _
ByVal x As Long, ByVal y As Long, ByVal cx As Long, _
ByVal cy As Long, ByVal wFlags As Long) As Long
Private Declare Function GetSystemMetrics Lib _
"user32" (ByVal nIndex As Long) As Long

Const SM_CXSCREEN = 0
Const SM_CYSCREEN = 1
Const HWND_TOP = 0
Const SWP_SHOWWINDOW = &H40


Private Sub Form_Load()
Dim cx As Long
Dim cy As Long
Dim RetVal As Long
' Determine if screen is already maximized.
If Me.WindowState = vbMaximized Then
' Set window to normal size
Me.WindowState = vbNormal
End If ' Get full screen width.
cx = GetSystemMetrics(SM_CXSCREEN) ' Get full screen height.
cy = GetSystemMetrics(SM_CYSCREEN)
' Call API to set new size of window.
RetVal = SetWindowPos(Me.hwnd, HWND_TOP, 0, 0, cx, cy, SWP_SHOWWINDOW)
End Sub