Any bigger than about standard is a problem for me. I have relatively large hands, and I still own decks that I find difficult to shuffle. A lot of tarot readers are women, who tend to have smaller hands, and larger decks can be a problem for a large proportion of readers.
That said, there are less usual methods of shuffling, like the overhand method, that work even with big decks, and some decks have such lovely art that it would be a shame to reduce the size. The Japaridze tarot, for instance, really needs to be as big as it is, even though it's a nightmare to shuffle. And I think most people know of decks hurt by how small they are--the 1001 Nights is a good example.
So I guess it's not a disaster if a deck is big, provided the artwork is complicated enough to justify it, but in general smaller decks are better.