Emily
I struggle to shuffle my Liber T which is a standard size LoScarabeo so anything over that size I can't shuffle but, to save my fingers, learned how to shuffle top-over-top so unless the deck is very small I shuffle that way anyway.
Riffling (my preferred method) does NOT work with thick Schiffer stock.I have the feeling the OP wasn't talking about riffling. If I shuffled by riffling, I could probably shuffle dinner-plate-sized cards. Hand size really has more to do with holding the deck to shuffle it, IMO.
I voted 14cm because I have the DruidCraft and I can shuffle them, albeit slowly and somewhat clumsily. It's not just the size of a deck that makes it harder to shuffle for me. Sometimes a deck that is thick is just as uncomfortable. For example, the Flornoy Noblet is small, but it's probably the thickest of all my decks and it has pointy corners (thinking about trimming those). It's a little awkward to shuffle.
Riffling (my preferred method) does NOT work with thick Schiffer stock.
But my response related to overhand as that was clearly the issue.
When I asked the question, I kinda assumed that everyone riffles the same way, by holding the cards across the widest point, like this (although maybe not in-the-air-riffling with bigger decks) https://www.youtube.com/watch?v=uW8zMwJF5ys I wasn't counting table riffling at the corner of the cards because I take this as a sign that the deck is too big to hold.
I also assumed that if people didn't riffle, that they still held across the widest point of the card, because you don't always have a table to hand? Or maybe you guys do!?! So when I asked about hand size, I thought thumb to middle finger length would be the key measurement.
I have riffled that perfectly easily. I wonder what's up with your copyI doubt very much if riffling would work well with the Illuminati either, since the cards feel like styrofoam beer coasters. I'm sure I'd have creases across the middle of most of them if I tried.
They do cost considerably more than a deck of playing cards so I handle them with a lot more care. I used to think riffling looked flashy and sophisticated, but now I'm just pragmatic about shuffling.