IPv6 minimum and maximum prefix lengths
The minimum prefix length can theoretically be configured to be /0 but much like a subnet mask of 0.0.0.0, it would not be used unless you are specifying a default route, such as
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.