The leg shown in the photo is "shank on", but that's optional with the meat cutter. This leg was 21 inches long, 11 inches wide, 4-1/2 inches thick and wighed 11-1/2 pounds. It is more common for legs to be sold asl "half legs" due to size and weight.
More on Lamb / Mutton Cuts.
Buying: These can be found in markets serving communities from countries that use a lot of lamb: Turkey, Armenia, Middle East, North Africa, Pakistan, etc. Legs on display will generally be half legs - you usually have to ask the butcher for a whole one.
Cooking: Leg meat is well exercised so it's both flavorful and tough, so it's generally cooked by a fairly long roasting, in the oven, on a covered grill or on a spit.
Yield: The 11-1/2 pound whole leg in the photo at the top yielded as per the table below. The percentages don't add up to quite 100% due to liquid loss, measurement rounding and other factors, but it's close enough for government work, so you engineers just try not to be anal about it.