The National Mah Jongg League (NMJL) is the governing body for American Mahjong. Every April, the NMJL releases a new card with that year's standardized hands and rules. 

Dimensions:

6.12" x 4.75" (This is the larger size offered by the NMJL)

Many hostesses prefer to have at least 4 cards, so they can host a full table without their guests needing to bring cards.