You can change the Advanced setting that automatically converts A4 to Letter in Microsoft Office Excel.

1. Open Microsoft Office Excel.
2. Click the File tab.
3. Select Options from the list.
4. Select Advanced in the Excel Options window.
5. Scroll down to the General section.
6. Clear the Scale content for A4 or 8.5x11" paper sizes option.
7. Click OK in the Excel Options window.
8. Restart Microsoft Office Excel.