Large for art's sake, less large for convenience's sake. It depends on whether you feel the art is more important, or that ease of handling is more important. If you think that the art is more important, by all means make the deck as large as you like, but lots of people won't use it, and those decks will be just pretty miniature portfolios, looked at occasionally but
rarely used.
If, on the other hand, you are making a deck for people to use, and the art is subordinate to that, then make a deck slightly smaller than the standard RWS. I have over thirty decks now, and I have to say that the one that feels best in my hand is the Ator Tarot by Aeclectic's own Rota. The dimensions on the Ator are 4.25" long by 3" wide. Absolutely perfect for me, and the cardstock is good, too. You'd have to ask Rota about the stock. You can get a good idea of this size card by cutting one out of an index card; you'll see how easily it fits into your hand.
Bob