Property height

The height property is used inside spacer blocks to configure how much whitespace it should represent, this will default to 50px.

{
    "from" : "info@example.com",
    "subject" : "Email with a single spacer",
    "content" : {
        "blocks" : [ {
            "type": "spacer",
            "height": 50
        } ]
    }
}

For more information and more examples, please check the documentation of spacer blocks.