Hey there Noel !
Your questions regarding shrimp and prawn are actually very good questions, indeed. The terms used to describe shrimp size—small, medium, large, jumbo, colossal—mean different things in different locations, and the jargon has no industry regulations. The more universal technique measures shrimp by the count, or number. If the shrimp are “16–20s,†that means there are 16 to 20 shrimp per pound, regardless of the label’s large, extra-large, or jumbo designation.
Hundreds of shrimp species swim in the seas, and some have minute differences we would never notice on our plates. The greatest variation may exist between the broad categories of warm-water, cold-water, and freshwater shrimp. Warm-water shrimp grow larger, but tend to taste less sweet than their cold-water cousins. Freshwater shrimp are usually farm-raised and prized for their size. Regardless of raw shrimp’s color, which can range from white to yellow to brown to striped, all shrimp turn pink when cooked.
Once again, the words “Jumbo, Large, Medium, Extra-Large” have no real meaning when one buys shrimp. These words are used in the retail trade and there is no standard to apply these names to a definite size of a shrimp.
When shrimp are sold commercially, the sizing is done by counts. A count is the number of shrimp in a pound (USA). If the country uses the metric system, it would be number per kilo. In the US, the common sizes would be 10/15 count, 15/20 count, 21/25 count, 26/30 count and 31/35 count. When the size gets smaller than 40 per pound, the counts will jump from a difference of 5 per pound to 10 shrimp per pound.
There are shrimp larger than 10/15 count and these would be graded accordingly. By law, there is a difference between shrimp and prawns. Prawns are from fresh water and shrimp are from salt water.
Prawns:
While some believe the term “shrimp” refers to the salt water crustacean, while “prawn” covers the fresh water version, in truth, prawns migrate, like salmon, from salt water to fresh water. Technically prawns are a different species than shrimp (with narrower bodies and longer legs), but the term is used loosely to describe any large shrimp as well.
And there you have it Shrimp and Prawn 101. 
Regards,
B-man 