My tank is 5 feet long, 18 inches front to back, and 26 inches high, whereas a standard 120g is usually 4 feet long, 24 inches front to back and 24 inches high. The standard arrangement would give more comfortable "swimming room," but the extra two inches of height allows more depth of substrate and height for climbing apparatus, for the crabs.
If you haunt the aquarium shops, remember to ask them for second-hand tanks and "leakers." There's really no need to buy a new tank for crabs, and large new tanks are expensive!!! I highly recommend checking craigslist, OfferUp, and other kinds of online second-hand markets; it's amazing what some people are just hoping to get rid of. I got a 75g back when we lived in Arkansas; the woman just wanted it gone, said if I could pick it up and get rid of it she would just give it to me.