Technically Prawns are almost the size of small 1/2lb lobsters missing the huge claws out front, but I have come to find in most places it is an extra large shrimp 15-20 or bigger. When you purchase shrimp you will notice they are in counts, check at the supermarket next time you are there. Look for the smallest number and that you will generally find is about the size of a prawn. I have also seen "prawns" the size of "micro" shrimp (45-60 counts)....really all depends on who is writing the menu and how technical they are about shellfish.