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.
Tarot size is my absolute favorite. I have the Thoth and Tarot of the Holy Light in both sizes and as much as I love the larger size, the regular size gives the decks a normal look and feel especially when reading for others. A large size is nice to look at but a regular tarot size seems to be more usable and practical.
ETA: And yes, you do need more room and yes I really like the poker size Morgan Greer.