I think that excel likes to remember the window state when you close it.
So if you close your full-screen workbook last (then close excel), when you
reopen excel, the next workbook will be full screen.
I don't think you can control this behavior.
About the best you could do is add some code to each workbook that tells excel
how to treat the windows when it opens that workbook.
this maximizes the application, but makes each window in that workbook normal
(not maximized, not minimized).
Option Explicit
Private Sub Workbook_Open()
Dim myWindow As Window
Application.WindowState = xlMaximized
For Each myWindow In Me.Windows
myWindow.WindowState = xlNormal
Next myWindow
End Sub