The version of WORD ( Microsoft Office Professional Plus 2010 ) I have, does not have an "Index Card" ( 3"x5") size either.
In fact, no "Index Card" entry.
It does have at least 4 different "Postcard" sizes,
-- and the standard letter size is listed 8 different times, under various names/headings
The way I see to do it, is to select the "More Paper Sizes", and set up a custom size