Hey, thanks for asking! I actually did a bunch of math and planning in a giant spreadsheet to make sure I could cover everything while maintaining the same probability spread as the tables in the game. It's been a while so I can't remember specifically, but I'm pretty sure I made it 120 cards because that number meant I didn't have to make any concessions.
As far as the size of the cards, they're the same size as the official Ker Nethalas cards. I think they're poker size? But the same sleeves should work for both.