They take about 6 years to reach market size. A surf clam can grow up to 9 inches (22.5 cm), but most rarely grow over 8 inches (20 cm.)
They are harvested by dredging.
Because they are large and tough, they are rarely sold whole: they are usually sold canned, and chopped up for cooking.
Weinberg, James. Atlantic Surfclam. Northeast Fisheries Science Centre: Woods Hole, Maine. January 2000.