How to set MTU for nodes?

Hello, Joshua.

Thank you for launching the bug.
We are updating "Reference Architectures" guide as soon as possible.

About your case:
you can download default configuration, edit it and uploade it back
as described here
http://docs.mirantis.com/openstack/fuel/fuel-5.0/reference-architecture.html#id48
The place to set "mtu" parameter is "network_scheme:->interfaces:->eth0:->mtu:".

Below is an example:

<code>
network_scheme:
  endpoints:
    br-ex:
      IP:
      - 172.16.0.7/24
      gateway: 172.16.0.1
    br-fw-admin:
      IP:
      - 10.20.0.7/24
    br-mgmt:
      IP:
      - 192.168.0.7/24
    br-prv:
      IP: none
    br-storage:
      IP:
      - 192.168.1.6/24
  interfaces:
    eth0:
      mtu: '1234'
      L2:
        vlan_splinters: 'off'
    eth1:
      mtu: '4321'
      L2:
        vlan_splinters: 'off'
    eth2:
      L2:
        vlan_splinters: 'off'
</code>