What Should Prefix Delegation Size on the DHCPv6 Server Page Be?

Under Services > DHCPv6 Server & RA > LAN > DHCPv6 Server, I've got the following (I think these are the default settings):

DHCPv6 Server = Enable DHCPv6 server on interface LAN
Subnet = Prefix Delegation (2600:8801:100:5700::/64)
Subnet Mask = 64 bits
Available Range = :: to ::ffff:ffff:ffff:ffff
Range = ::1000 to ::2000
Prefix Delegation Range = blank
Prefix Delegation Size = 48

That Prefix Delegation Size of 48 bothers me.  AFAIK, I'm getting a /56 from Cox.  Shouldn't the Prefix Delegation Size on the LAN's DHCPv6 Server page be a number between 56 and 64?  I'd assume /64 would be correct, but I'm mostly clueless.

Anyway, here's some supporting stuff:

On the  WAN interface:
IPv6 Configuration Type = DHCP6
DHCPv6 Prefix Delegation size = 56
Send IPv6 prefix hint = Enabled
Debug = Enabled

On the LAN interface:
IPv6 Configuration Type = Track Interface
IPv6 Interface = WAN
IPv6 Prefix ID = 0

Filtering Status > System Logs > DHCP for "prefix" gives:
```
Mar 2 11:44:45	dhcpd	83677	Unable to pick client prefix: no IPv6 pools on this shared network
Mar 2 11:43:11	dhcp6c	21421	create a prefix 2600:8801:100:5700::/56 pltime=86400, vltime=86400
Mar 2 11:43:11	dhcp6c	21421	IA_PD prefix: 2600:8801:100:5700::/56 pltime=86400 vltime=140733193474432
```
So, it looks like I'm getting the /56 from Cox.  Is that first message about "no IPv6 pools" perhaps related to the /48 on the Server page?