May be the following will help:

Press F5, Special, Last Cell, OK

You will see that last cell is Y65536.

Now try the following:
(1) Press the grey are above row 1 and to the left of column A to select the whole sheet.
(2) Format, Column width, 8.43, Enter.
(3) Press A to select whole column A, Format, Hide & Unhide, Unhide rows.

Let's say your last used row is 1000 and your last used column is Y. Do the following:

(4) In the Name Field write 1001:1048576, press Enter, right click the mouse and press Delete.

(5) In the Name Field Z:XFD, press Enter, right click the mouse and press Delete.

(6) Save the file.