How to use Style Settings

In this site: Style settings
it shows that we can use Style settings in our theme.
but when i add the follow code in my theme:

{
  "type": "style.size_panel",
  "id": "size",
  "label": "Size",
  "default": {
    "width": "25%",
    "height": "auto"
  }
}

it shows : Invalid schema: setting with id=“size” type is invalid

Style settings aren’t available publicly yet, they’re only available on the Flex sections developer preview.

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