Another MS Word problem: formatting page size

I want to print on some 5"x8" index cards. So I go to File/ Page Setup and select Paper Size: Custom; Width 5.0"; Height 8.0". Since my inkjet printer requires a minimum 1/2" margin, I set the margins at 0.5" all around.

But then when I go to print it says “The margins of section 1 are set outside the printable area of the page. Do you want to continue?”. If I say yes, it prints the 5x8" area center justified on an 8 1/2X 11" area, which is not what I wanted. Expanding (or contracting) the margins makes no difference. The only thing that works is setting the page size to one of the standard preset sizes like Letter or A5, and then setting the margins to narrow the printable area to 5x8". Either Word, or my printer, refuses to believe that there is such a thing as a custom 5x8" paper size. Perversely enough, it will accept an 8x5" card (landscape mode). But not portrait mode. Am I doing something wrong, or is it a driver problem?

My WAG is that the printer’s grippers are positioned in such a way as to make feeding paper below a certain width impossible or unreliable, and apparently 5 inches is under that dimension. So the printer’s drivers don’t allow you to set that width.

You might test that theory out by seeing if it will accept 6 inches, 7 inches, etc. and seeing where it works.

But why bother? Why not print in portrait mode, since that works, or lay out a standard 8.5 x 11 inch document with nothing in the L-shaped area outside the 5 x 8 inch area, and print it as though it were an 8.5 x 11 inch document?