So you would want to change the mpls mtu on the PE and P routers to accommodate a full ethernet frame plus the two mpls labels and any other additional info, like control words or vlan tag you might be transferring across the backbone, a value like 1514 would work, but if you ever decide to do straight ethernet to ethernet l2vpn with QinQ, then you would be up to 1530. If you maybe did serial links in the future, you'd maybe be up to 4480-4490.
In most SP environments I've worked it, its standard to set the mpls backbone mpls mtu to 9100 or 9192 at install time to remove any issues with the backbone mtu.
Mpls mtu only affects handling of labeled packets, not unlabeled packets.
