How big do hermit crabs get?

[newfishguy]

I have eight hermit crabs (blue leg and scarlet) in my 15 gallon tank. I recently got some empty shells for them to change into. I am wondering how big these guys will get. Should I keep getting larger empty shells?

 

[sinner's girl]

keep getting larger shells, always have an assortment of sizes for them.

 

[wax32]

Blue legs and scarlets don't get too big, a large Astraea snail sized shell is plenty for them, although most seem to prefer weird shaped shells. Cerith snail shells are favored by blue legs.