A question about IPV6

So I'm trying to set up a pfsense router via the Oracle Virtual box application. For IPV6, I set up the LAN interface for the pfsense router virtual machine with a Unique Local Address which requires that I enable Network Prefix Translation (NPT) within pfsense for it to work, since it is equivalent to a privately assigned IPV4 address that also uses NAT. In the pfsense interface, I apparently have to input a destination IPV6 prefix to get it to work. Does anyone know which IPV6 address I'm supposed to be using to get this done? Is the prefix supposed to be /60 or /64?