I prefer the more common size 2.75 X 4.75in / 7 X 12cm.
If the art is really complex, then it's easier to see on a Jumbo size. But then the cards are more difficult to handle. Some people have small hands that can't shuffle a big deck easily. And no matter how big someone's hands are, those big cards take up a lot of space on a table.
My favorite size is a compromise between the two. It's 3 by 5 inches.