IPv6 minimum and maximum prefix lengths

The maximum prefix length you can use for an IPv6 address is /128 which essentially is a host address, much the same as using a subnet mask of in IPv4.

The minimum prefix length can theoretically be configured to be /0 but much like a subnet mask of, it would not be used unless you are specifying a default route, such as ::/0.

Link-local addresses must by definition have a minimum prefix length of /10 according to this draft RFC.

Otherwise, you can configure whatever prefix length you like. There are no restrictions inherently configured on a Cisco IOS device. Other operating systems may limit what prefixes you can use.