I love large cards, so I've learned to adapt and shuffle them in a combination of three ways (like Grizabella, I want to feel "in the groove" when shuffling the cards):
(1) I have the cards in two piles, held in my hands like this [][]. I hold them close, as if doing a regular shuffle, however, let them fall, one on top of another, on the table, then push them together. Pick them up and do it over and over again. It is practically like a riffle, just a bit looser.
(2) the way nisba suggests, i.e., turned 90 degrees "...use a longer, slower stroke-movement of pull, pull, pull, like taffy.
(3) another way is where I have both of my hands palm up, cards in my right hand and my left hand empty, and I slide a few cards into my open left hand, and keep doing that over and over, card(s) on top of card(s) until all the cards are transferred and mixed from my right hand to my left. []~[]
The cards are thoroughly mixed after doing and/all of the above. I wish I had a video camera to show you --it's quite easy with a little practice.