If you are using 3.99y, in the MainMenuButton.vb code at line 264 is where it sets the form size to fill the screen less the main menu
If MenuPosition = AdvancedHMI.Controls.MenuPos.LeftOrTop Then
If pf.width < pf.height Then
'* Menu is on left
f1.Size = New Size(System.Windows.Forms.Screen.PrimaryScreen.WorkingArea.Size.Width - pf.Width, pf.Height)
f1.Location = New Point(pf.width, 0)
Else
'* Menu on Top
f1.Size = New Size(pf.Width, System.Windows.Forms.Screen.PrimaryScreen.WorkingArea.Size.Height - pf.Height)
f1.Location = New Point(0, pf.height)
End If
Else
'* Menu on Right
If pf.width < pf.height Then
f1.Size = New Size(System.Windows.Forms.Screen.PrimaryScreen.WorkingArea.Size.Width - pf.Width, pf.Height)
f1.Location = New Point(pf.location.x - f1.Width, 0)
Else
'* Menu on Bottom
f1.Size = New Size(pf.Width, System.Windows.Forms.Screen.PrimaryScreen.WorkingArea.Size.Height - pf.Height)
f1.Location = New Point(0, pf.location.y - f1.Height)
End If
End If