hi stacey
The common thing to do is to make the art 2-3 times larger than the finished card, but it is not a rule. It just helps with getting the details in and having a smoother looking finished product.
That said, I didn't do that with my deck. I made the art originally to be the finished size of the card. It was really difficult to paint the small details - I had to use the smallest paintbrush I could buy, and hold my breath at times to avoid shaking when painting stuff like details or faces. When I do the next one, I intend to work larger.
I did end up shrinking the images ever so slightly for the cards. Hardly anything at all, they are still very near to actual size. I didn't want to shrink them at all as I wanted all of the details in the art to show, but I wanted the finished cards to be easy to shuffle for us small handed folk. And the shrinking was so minor that all of the details are still perfect.
Good luck with your deck!