I found getconf HOST_NAME_MAX to return 255 on my Sol 10 box, so I'm assuming that that is the max length of a hostname in Solaris 10. Can this be expanded? What about for ipv6?
The length of a hostname is not related to IP at all. It's a name, so only name resolution protocols matter. DNS, for example, limits the length of a single label (dot-separated part) to 63 characters and the length of a full domain name to 253 characters.
Either way, if you do need a hostname longer than 255 bytes, you have bigger problems.