There are a rather large # of differant hermits in the hobby. I have seen standard blue, red, scarlet, also large neon blue, ...etc.
With my
scarlet hermits I do not really worry about shells, over the years I have built up a number of empty ones in the tank. They will pick what they want when they want it.
For the neon blue hermit I had(golf ball size) I bought several larger shells and placed them in the open on the sand bed were they would not be burried with sand. Over the years I had him he switched shell on his own, seemingly more for his own taste than size.