If you're worried Hermie's stuck in a shell too small for him, you're worrying over nothing if his shell is unpainted. Hermit crabs are choosy home shoppers, careful to pick shells meeting their specific needs. In most cases, you should leave your crab be and allow him to come out on his own.
If Hermie's not interested in coming out of his shell, it may be that's he's perfectly content where he is. Hermit crabs in the wild are known for their improvisational skills when they search for housing, using bottle caps and miniature alcohol bottles for shelter if they can't find appropriate housing. If his shell looks too small and he's acting a bit lethargic, he may be getting ready to molt.
Pet shops often sell crabs in painted shells, a controversial practice many hermit crab enthusiasts decry. If Hermie was wearing a painted shell when you brought him home, it's possible he's stuck in his shell. One of the problems with painted shells is the potential for a crab to become glued inside. A variety of merchants don't sell hermit crabs in their "native" garb, but instead force hermit crabs to change into painted shells to improve their saleability. If a crab is forced into his painted shell before the paint inside is dry, he'll get stuck.
If Hermie's stuck in a painted shell, it could have fatal consequences. Crabs in such conditions can't eat or move properly, resulting in a unpleasant death. Pulling him out is not an option, as he'll be torn apart in the process. On the Crab Street Journal, one crab enthusiast detailed the process of clipping her crab out of his painted shell, an ordeal that took almost three hours. Cutting a stuck crab out of a painted shell is a risky proposition, as his legs, eyes and abdomen are all vulnerable to injury.
The Proper Shell
If Hermie's reluctant to give up his shell, the best you can do is provide him with a nice variety of unpainted shells to consider trading up for. If Hermie's just one of several crabs in the crabitat, providing multiple shells of the same variety may prompt him to trade up and reduce the chance of shell fights with his pals. Avoid shells with holes, cracks or other broken parts, as they can affect Hermie's humidity level. Don't be tempted to add a painted shell as a choice for Hermie, even if it's dry or hasn't been painted on the inside; the paint will chip off and potentially hurt him if he eats the chips. Hermit crabs will give up their limbs for their shells, so never try to force your crab out of his mobile home.