Hi Ken,
Excel 2021 saves the size and position of the last workbook window you close. The fix is to force Excel to save a maximized window as the last one closed.
Close every Excel window. If any refuse to go away, open Task Manager and end any EXCEL.EXE you still see. Now open just one workbook. Click the Maximize button so it fills the screen. Do not open anything else. Close Excel using the X on that window so the maximized window is the last one to close. Open Excel and your workbooks again. They should now open maximized instead of that horizontal view.
If the behavior returns, repeat the same steps once, since it only sticks for the last closed Excel window.