I asked Andrei about your card size, and he said that you haven't hit a limitation. The maximum height is 100, so creating a 10x21 card in a 10-column layout should work.
The only maximum card size that I'm aware of is the restriction that if a card has more columns than the layout to which it's assigned, the card will not appear on the browse page. For example, a 6x6 card cannot appear on a 3-column layout. In your case, the 10x21 card should appear as a very tall card on your 10-column layout. I've asked someone who works on cards and layouts to chime in.
I asked Andrei about your card size, and he said that you haven't hit a limitation. The maximum height is 100, so creating a 10x21 card in a 10-column layout should work.