Sure, I can help you with that! Here are the steps to choose A3 paper size in Excel:

1. Open your Excel workbook and click on the "Page Layout" tab in the ribbon at the top of the screen.
2. In the "Page Setup" group, click on the small arrow in the bottom right corner to open the "Page Setup" dialog box.
3. In the "Page Setup" dialog box, click on the "Page" tab.
4. Under "Paper Size", select "A3" from the drop-down menu.
5. Click "OK" to apply the changes.

That's it! Your Excel workbook should now be set to A3 paper size. Keep in mind that changing the paper size may affect the layout of your worksheet, so you may need to adjust the margins or scaling to fit your content properly.