Count product variants in an array using javascript/GTM

I am setting up a variable in GTM to return a count of the number of sizes in-stock (regardless of the color).

In the example below (pulled from the dataLayer) there are 2 colors and 2 sizes for each color (total of 4 sizes). But, the first size is out of stock. So, in this case the output would be 3, since 3 of the 4 sizes are in stock.

How would I set up the JS to make this work?

I really appreciate the help!

{
"product": {
    "variants": [
        {
            "colorID": "03",
            "colorName": "MEDIUM DENIM",
            "sizes": [
                {
                    "sizeID": "10",
                    "sizeName": "33",
                    "available": "false",
                    "price": "24.0"
                },
                {
                    "sizeID": "1",
                    "sizeName": "24",
                    "available": "true",
                    "price": "24.0"
                }
            ]
        },
        {
            "colorID": "06",
            "colorName": "LIGHT DENIM",
            "sizes": [
                {
                    "sizeID": "1",
                    "sizeName": "24",
                    "available": "true",
                    "price": "24.0"
                },
                {
                    "sizeID": "2",
                    "sizeName": "25",
                    "available": "true",
                    "price": "24.0"
                }
            ]
        }
    ]
}

}