根据您的注解，您似乎拥有以下数组：

[
    [
        "pa_size" => "m",
        "pa_color" => "blue"
    ],
    [
        "pa_size" => "xl",
        "pa_color" => "yellow"
    ],
    [
        "pa_size" => "m",
        "pa_color" => "blue"
    ]
]

您需要做的是调用array_map（documentation）来获取大小，调用array_unique（documentation）来获取唯一值。
一旦获得了这些唯一值，就可以选择使用array_filter（documentation）按大小获得乘积。
下面是一个例子：

// define the products
$products = [
    [
        "pa_size" => "m",
        "pa_color" => "blue"
    ],
    [
        "pa_size" => "xl",
        "pa_color" => "yellow"
    ],
    [
        "pa_size" => "m",
        "pa_color" => "blue"
    ]
];
// get just the "pa_size" values
$productSizes = array_map(function($key, $value) {
    return $value["pa_size"];
}, array_keys($products), array_values($products));
// get the unique "pa_size" values
$uniqueProductSizes = array_unique($productSizes);

// loop over the unique "pa_size" values
foreach ($uniqueProductSizes as $key => $value) {
    // get every product by the "pa_size"
    $productsBySize = array_filter($products, function($innerKey) use ($value) {
        return $value === $innerKey["pa_size"];
    });
}

小提琴：https://onlinephp.io/c/d637f

你的解决方案工作，但不适合我。我不知道哪里是问题。下面的片段和例子回应。

add_action( 'woocommerce_before_shop_loop_item_title', 'variations_loop' );
function variations_loop(){
    global $product;
    if ($product->get_type() == 'variable') {
        foreach($product->get_available_variations() as $key) {
            $variation = wc_get_product($key['variation_id']);      
            $varation_size = $variation -> attributes;      
            var_dump($varation_size);           
        }
    }
}

array(2) {
  ["pa_size"]=>
  string(1) "m"
  ["pa_color"]=>
  string(4) "blue"
}
array(2) {
  ["pa_size"]=>
  string(2) "xl"
  ["pa_color"]=>
  string(6) "yellow"
}
array(2) {
  ["pa_size"]=>
  string(1) "m"
  ["pa_color"]=>
  string(4) "blue"
}