Okay When first open an Excel you roughly see this view
Shows at the top left of the screen this is 100%. Most People resize and shift the position on the screen and save this view, so it close to be centered on the screen.
To set this Position, you option Click on the Green stop light button This will expand or contract to previous position and size, Repeat. Doing this set as default size and position for this document.
Now Save document with the correct name (even if you haven't filled out anything in sheet yet). From now on everytime you open the document and option click the green button will return to your preset. (Notice the + sign in the green button.)
Any Modern Mac OS there is a setting called full which expand the item to fill up the entire screen. To get to it simply click on the green button. The Buttons look like this when you hover over then without holding the option key down:
Notice the two arrows at 45º angle. In this mode, even the menubar is hidden and only shows if you move the mouse to the top of the screen.
NOTE: if you set up the document and save that document, it will always open to that size and position. Occasionally it will revert to covering 1/3 the top left of the screen just click on option Green button will shift back in righ size and position.
Opening a new blank document will always show 1/3 of the top left of the screen.
Hope this helps.
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