You can set the Maximum Transmission Unit (MTU) value in bytes for router advertisements on the IPv6 interface to a value between 1280 - 4294967295 bytes.
This example sets the router advertisement MTU value to 12000 bytes for VLAN 50:
System(su-config)->interface vlan 50 System(su-config-intf-vlan.0.50)->ipv6 nd ra mtu 12000 System(su-config-intf-vlan.0.50)->