Paper Size keeps changing to Letter Index Card

I am on a Mac with OS Catalina and using Office 365. When I print a Word document and select paper size = US Letter > US Letter, the document prints fine. However, when I save the document and re-open it, it has defaulted back to US Letter > Index Card Letter 8.5 x 11. I could live with this, except that this setting doesn't allow duplex printing. I think the problem is that there are 3 options under US Letter and the first one is "Index Card Letter 8.5 x 11". I suspect that I could solve the problem by deleting the Card option or by moving the US Letter option to the first position. Anyone have any ideas?