I don't know if there are any big crabs around for fishkeeping, the ones i've seen are all around the same size (like red clawed crabs and vampire crabs). They all need access to land though. Maybe someone else will know of one.
I don't know much about them, but an LFS near here sells Moon crabs that are at least 4 inches wide by 3 inches tall. This is much larger than any Red Claw or Fiddler I've ever seen. I have no idea if this is normal or how they were bred or whatever. But yeah, any crab is going to need access to land.
Not something you would find in an aquarium usually but if you want big you can't really go past a Mud Crab. Not sure if they need to get out of the water or not, (think they might) but they get to 10" across the carapace & have claws bigger than a mans hands that are easily capable of removing a finger or two if you get careless with them. Don't know how you would go getting them in Canada alive though, they are native to Australia.
I didn't mean that every crab needed access to land, just the ones I know of in fishkeeping. You could ask your LFS if they know of any or you could go with the next best thing and get a lobster (a different species than you already have).
Yaa, just want something ****, red claws are probably the biggest, well get the biggest around here anyways, for as easy as they are to find anyways lol
I got a panther, but they are pretty small themselvs too.
Maybe get 1 marbled cray fish! But make sure it is a true marbled crayfish that way it self clones itself! You can fit 50 in a 20 gallon, they don't need access to land and grow 12cm. Also do you live in vancouver? If so there's a guy who give sthem for free. I'll contact him for you if you live near here.