A full-grown Procambarus alleni including claws may be 6-7" in length. An ideal setup for one crayfish would be a 20 gallon long tank (floor space is more important than height for them) with some good hiding spaces in the form of rock 'caves', driftwood, or flowerpots. They are not compatible with fish, so only add fish you consider expendable; same goes for plants. Crayfish will eat almost anything, including most types of fish food, small fish, snails, plant matter, vegetables, and so on. Temperature is not too important, so long as it does not get extremely hot or cold. Otherwise, just keep the water clean and well-aerated and your crayfish will be happy!
Be aware that crayfish are excellent climbers and can escape tanks easily if there is any opening. A secure top is very important.