Corydoras vary in size from the little pygmies that I like so much at 1.3 to 1.4 inches up to the largest of them at nearly 4 inches. The most common ones are, as Inchworm said, in the 2 to 3 inch range as adults. The ones you often see in the LFS are young fish that are maybe 1/2 to 2/3 of their adult size. That will give you a rough way to judge ultimate size if you see something you like and don't take the time to research them before buying.