Hermit crab's shell size

[haihi]

I have two hermit crabs and am wondering how to determine that the new shells that I've purchased for them are of suitable size. I am aware that the opening of the shell must be slightly bigger, but how bigger must the new shell be. Moreover, does the height of the shell is of importance? Is it alright for me to have a shorter shell but one with a bigger opening or must the shell be taller as well. Its quite hard to get new shells in Singapore and am thinking of getting from online shops overseas. I have two ruggies thus am looking at turbo shells.

Thanks in advance

[cartoonshrimp]

hihi bro I used to keep hermit crabs as for my case I put at least 3 shells for my hermit crabs to choose because hermit crabs tend to try and error for the shell. So just put a few shell(around the size) will do. =)