A: Exactly what we print on* is not necessarily relevant to you. Check with the printer. Any game printer will have an idea of what are the various cardstocks which are appropriate for a game. They should be able to send you samples of printed cards on the different types of paper, and the cost of each one (or how the cost of a given paper will affect their quote to you).

Note that one important question is whether there is a light-barrier layer sandwiched in the middle. This is to ensure that you can’t see through the card if someone is holding it in their hand, and they’re backlit. It costs more than comparable cardstock without it. You’ll have to decide whether your game requires that.

Regarding size of the paper, ask each printer whether they have a certain cardsheet size that is cheaper, perhaps because they print other games with that size deck. That may affect design factors like how many cards you want/need in your game.

For example, when we did the second run of Fluxx, one of the factors in changing the card size was that our printer at the time printed a lot of UNO and MtG decks, which had a certain size card and number of cards in the deck (84). Now we work with a different printer, and the best number of cards is 100, so Andy worked out some new cards. Now he designs new games with that deck size in mind.

* Here is a question about the actual paper used for our US print runs.