Dear Blue Lobster:
I bought a blue lobster from WalMart about 3 years ago. He is now about 10 inches long (tail to claw) and still growing. How big do these get?
Dear Gentle Sir:
Without knowing the species, it is hard to say. Typically, there are only two species sold at retail chains in North America: Orconectes immunis, which reaches around 14cm (5½ in.) or Procambarus alleni, which grows slightly largers at 15cm (6 in.). Since your cray is roughly twice the length, it is possible that it's an Australian variety, as Australian species often exhibit insular gigantism. Whether it's a Cherax destructor, fabulosis, quadricarinatus, or tenuimanus, or another less common species, it could easily grow upwards of 34.5cm (13½ in.).
Since your cray is quite large and will likely grow larger, you should have a twenty gallon tank at a minimum, if not twice that. Considering the better conditions the cray will recieve in your tank, and the possibility that there are growth-inducing chemicals present in the water itself (from birth control pills, industrial runoff, etc.), your cray would be safest in a one hundred gallon tank. This would allow him or her to grow to an impressive 45cm (almost 18 inches!) and still feel unstifled. You can also create quite the reproduction of a natural environment with the extra room, including logs, rocks, shallows, and substantial flora.
In the future, when asking for help regarding a specific cray, include some documentation that might help identification. This includes some photographs from different angles, especially the cray's underside, place and date of purchase, and the common or scientific names and any other information given at the store. One may be able to deduce from some of the less direct evidence what family or genus the cray is, if not the species. This can help make any advice regarding your cray more accurate and therefore more beneficial in the care of your little crustacean friend.