[foxfish]

If you want to correctly cook your crustaceans then you need as much boiling water as possible, heated by the biggest flame you can manage. The bigger the volume of water the less affected the temperature will be when you place in the crabs. You dont want the water to stop boiling when you drop in the crab as "A" it is not very humane "B" your timing to cook the beast wont be accurate.
I would recommend a minimum size pot of one & a half gallon but the bigger two gallon stock pots are better. I have never found this size stock pot difficult to find, readily available in most cook shops or hardware shops.