overlay-amd64-generic/scripts/disk_layout.json

{
  "_comment": "See http://www.chromium.org/chromium-os/building-chromium-os/disk-layout-format",
  "parent": "legacy_disk_layout.json",
  "layouts":{
    "common":[
      {
        "num": 3,
        "fs_size":"2000 MiB"
      }
    ],
    "usb":[
      {
        "num": 3,
        "size": "2048 MiB"
      }
    ],
    "factory_install": [
      {
        "num": 3,
        "size": "2048 MiB"
      }
    ],
    "recovery":[
      {
        "num": 3,
        "size": "2048 MiB"
      }
    ],
    "4gb-rootfs": [
      {
        "num": 5,
        "size": "2 MiB"
      },
      {
        "num": 3,
        "size": "4096 MiB",
        "fs_size": "4000 MiB"
      },
      {
        "num": 1,
        "size": "2432 MiB"
      }
    ]
  }
}