/28 for any ISP having several IPv4 prefixes and committed to deploy 6rd would be IMHO a good choice. In practice, /60s to customer sites is quite sufficient, at least in the short term.
No, /60s to customer sites is not sufficient. It breaks the IPv6 model of fixed size site allocations which assumes that all sites will have /48 assignments unless they are private residences in which case they will have /56 prefixes. This uniformity is essential in order to allow people to innovate with IPv6. We should not force 6RD ISPs into making this kind of mistake, especially since 6RD is only a transition measure, and if an ISP gets /60 encoded into all their systems and processes, then they will be stuck with it long after 6RD has disappeared. That would be an IPv6 equivalent of the swamp, i.e. a short sighted, short term decision with negative long term consequences. --Michael Dillon