This isn't set by Excel. This is part of the Windows setup.
- From your desktop, right click and select "Properties
- Select "Appearance
- Select "Advanced" and from the "Item" dropdown, select "Scrollbar"
- In the "Size" box, choose a font size
- Click on "OK" and "OK". After you click on the second "OK", you might
get a Windows message as it applies the revised settings.