Printing on 5X8 index cards

It can be done.  While you can certainly create custom paper sizes and use precut cards, I suspect you will find it simpler to print on letter sized cardstock (2 to a page) and cut them afterwards.  The advantages to doing it that way include borderless printing, a lower likelihood of paper jams, and simpler configuration.

How do I create custom sizes on the 5100?  I already have the 5X8 precut cards.

It may simplify things to use A5 (5 7/8 × 8 1/4) or B6 (4 7/8 × 7). If neither of those is close enough...
- You create the custom size in Word using "Page Setup...". You may need to specify the paper source as MPF.
- Then on the printer menu, set the MPF size to "Freesize" and type to one of the "Covers" types.
- When you print (use File > Print...) click the "Properties" button and set the paper type to the same type you chose in the printer menu or you'll get the ever popular "Load MPF Plain 1" message.