We put cultured marble on the shower walls in our bathroom. It cost approximately $20-25/sq.ft. installed. Real marble was, if I recall, marginally more but we liked the look of the cultured stuff better.
The biggest piece we needed was about 6 feet by 8 feet. Realistically the size of the slab is probably more likely to be limited by the ability to get it to and into the room than their ability to make the slab in the size you need. We had to remove a second floor window and take it in that way because it wouldn't go up the stairs and around the corners.