A leg of lamb can be boneless or bone-in. It is a very impressive and often expensive cut of meat that comes from the back haunches of the lamb. As a guide, you should look for an average weight of about 5 to 7 pounds. Don't buy a baby lamb and don't buy an old lamb that may be too tough when cooked. Also, ask the butcher to trim the outer tough layer of fat. But, don't remove all fat, you need some for flavor and to keep the meat moist and tender.