Lord_Dantes

add_action( 'woocommerce_before_add_to_cart_button', 'sizes_before_add_to_cart_button', 0 );
function sizes_before_add_to_cart_button() {
    global $product;

    // Only on simple products
    if( ! $product->is_type('simple') ) return;

    // When product sizes are available
    if ( $sizes = $product->get_attribute( 'size' ) ) :
    $sizes = explode(", ",$sizes);

    echo '<div class="lisatiedot__size__select" id="product-size-field">
    <label for="product-size">' . __('Valitse koko <br>') . '</label>
    <select class="select__size" name="product-size" id="product-size">
        <option value="">' . __("Choose your size") . '</option>';

    foreach( $sizes as $size ){
        echo '<option value="' . $size . '">' . $size . '</option>';
    }

    echo '</select>
    </div><br>';

    endif;
}
add_filter( 'woocommerce_add_to_cart_validation', 'filter_add_to_cart_validation', 10, 4 );
function filter_add_to_cart_validation( $passed, $product_id, $quantity, $variation_id = 0 ) {
    $product = wc_get_product($product_id);

    // Only on simple products and When product sizes are available
    if( ! $product->is_type('simple') && ! $product->get_attribute( 'size' ) )
        return $passed;

    if( isset( $_POST['product-size'] ) && empty( $_POST['product-size'] ) ) {
        wc_add_notice( __("Please choose your size.", "woocommerce" ), 'error' );
        $passed = false;
    }
    return $passed;
}
add_filter( 'woocommerce_add_cart_item_data', 'add_size_to_cart_item_data', 10, 3 );
function add_size_to_cart_item_data($cart_item_data, $product_id, $variation_id ){
    if( isset( $_POST['product-size'] ) ) {
        $cart_item_data['product-size'] = esc_attr( $_POST['product-size'] );
    }
    return $cart_item_data;
}
add_filter( 'woocommerce_get_item_data', 'display_size_on_cart_item', 10, 2 );
function display_size_on_cart_item( $cart_item_data, $cart_item ) {
    if ( isset( $cart_item['product-size'] ) ){
        $cart_item_data[] = array(
            'name' => __('Size'),
            'value' => $cart_item['product-size'],
        );
    }
    return $cart_item_data;
}

<form class="cart" 
							action="<?php echo esc_url( apply_filters( 'woocommerce_add_to_cart_form_action', $product->get_permalink() ) ); ?>" 
							method="post" 
							enctype='multipart/form-data'>
					<?php do_action( 'woocommerce_before_add_to_cart_button' ); ?>

					<?php
					do_action( 'woocommerce_before_add_to_cart_quantity' );

					woocommerce_quantity_input( array(
						'min_value'   => apply_filters( 'woocommerce_quantity_input_min', $product->get_min_purchase_quantity(), $product ),
						'max_value'   => apply_filters( 'woocommerce_quantity_input_max', $product->get_max_purchase_quantity(), $product ),
						'input_value' => isset( $_POST['quantity'] ) ? wc_stock_amount( wp_unslash( $_POST['quantity'] ) ) : $product->get_min_purchase_quantity(), // WPCS: CSRF ok, input var ok.
					) );

					do_action( 'woocommerce_after_add_to_cart_quantity' );
					?>

					<button type="submit" name="add-to-cart" value="<?php echo esc_attr( $product->get_id() ); ?>" class="single_add_to_cart_button button alt"><?php echo esc_html( $product->single_add_to_cart_text() ); ?></button>

					<?php do_action( 'woocommerce_after_add_to_cart_button' ); ?>
				</form>