It is true that potatoes can grow up to 5 and 9 pounds? My local restaurateur cite potatoes as large as 13 pounds.

Some varieties of Idaho potatoes such as the Ranger can grow to be 4-5 pounds in size. The largest size potato Idaho sells commercially is a 35 count, or 35 potatoes in a carton weighing 50 pounds. So that’s an average of just under 1 ½ pounds per russet. Most potatoes never make it into the consumer or food service marketplace over two pounds each. Casinos and steak chains like Morton’s Steakhouse usually order a pound potato or 50 count. 

Actual sizes are show in this carton count size guide:

Potato Size Guide