Start with somehow acquring the June TFH article as their is a captivating article on Nautilus husbandry.

You might also purchase Hanlon and Messenger's book on cephalopod behavior.
There are six described species the last time I checked. The average diameter of nautilus is about 20 cm.
As for the rest of the questions, no one really knows for sure as wild research on these animals is quite limited. In captivity they are fed a diet high in calcium such as crabs, shrimp, exoskeleton, etc. They live for about 3-4 years in captivity, although natural life spans are presumed to be much longer (15-20). It is not known whether they are solitary or not but I would guess that they are.
There are some interesting papers at cephbase.utmb.edu.
Greg