I have two huge (16-18") king crab legs and nothing big enough to put them in!
They should have been steamed already, unless your near a good market. Are the shells red?
I tend to warm/cook mine wrapped in foil in the oven a few mins with a splash of wine; another way is to get a roasting pot with the lid and drip pan and fill with water and pre-heat that full of steam in the oven.Or use foil if you don't have a roasting pan with lid.
No worries about breaking them tho, try to break at the joints.
I agree with Sam that normal practice is they are precooked and flash frozen in Alaska then shipped. That being the case, simply thaw and eat. If you want them hot re-steaming will make the meat tougher. King crab is saltier than other varieties so I would not salt them again (eg, if you use the meat fora crab cake).A nice old fashioned cocktail sauce will help cover any overt salty taste ... Try with a splash of lime juice.