Depends on what species of shell dweller you are referring to, really. A LOT of cichlids are called shell dwellers, and some of them get very large. The most common small shellie you will see is Neolamprologus multifasciatus, affectionately called "multis". They get around 1.5 inches to 2 inches, have a low bioload, and can be kept in as small as a 10 gallon. They prefer a fine gravel or sand substrate, so they can excavate. You will want at LEAST one shell per adult fish, plus several extras for juvies and territorial arguments. They prefer a higher pH, so you can put crushed gravel in the filter or mix it in the substrate to help buffer.
I have a colony of these guys in a 10 gallon, and they are a lot of fun.