Why do all my Excel 2016 files open to the size of a postage stamp?

I have not had to use this, but have heard this will resolve what you are experiencing.

1. Open Excel
2. Go to the View tab and click on New Window
3. Save the workbook

Not sure why it works, other than it most likely resets the window view settings.

I had to erase the following registry keys to get it to reset..

Replace 16.0 with whatever version of Office you are running e.g. 14.0, 15.0, etc.

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options  Pos
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options  Maximized

Deleting these two keys with Excel closed will force Excel to recreate the keys, which will resolve the issue.

Excel is trying to "help" you by remembering what you did last.

Here are a few things you could try:

1. Open Excel, without opening any .xls file (start it by Window+S and type 'Excel'), maximize it, and close.
2. If the above didn't work, resize the window by dragging the side or corner handles (don't click the maximize button) and close.
3. Run virus scanner?