http://answers.microsoft.com/en-us/office/forum/office_2010-office_install/excel-does-not-open-to-full-screen-when-opened/a7405b25-1f2f-4465-ba64-f80a8adaae64
Add this a macro in Personal.xlsb:
ThisWorkbook: Private Sub Workbook_Open() Application.WindowState = xlMaximized End Sub