rossignol said:5-6 inches is one huge crayfish. Are you sure it's not a lobster? Got away from someone's Sunday dinner?
I used to enjoy keeping crayfish in my native tanks. But one warning, they will eat anything. And that includes sleeping fish that it can catch. If it's small enough and slow enough to grab, the crayfish will eat it.
There are many species of crayfish that get to be 5-6". The largest crayfish is the giant tasmanian crayfish (Astacopsis gouldi) which can grow to almost 3' in length. Here is a picture of one of those beasts.

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