formulario no pasa valor de la variable a funcion

$product = wc_get_product( $post_id );
$product is passed to your function so it already exists, no need to find it.
$post_id does not exist in the function
$product is not used later in the function

$post_id = $post->ID;
$post_id is not used later in the function

If you need the $product_id, you would do:
$product_id = $product->get_id();

It should work without these two lines:

$product = wc_get_product( $post_id );
$post_id = $post->ID;

they don’t seem to do anything.

$product_id = $product->get_id();
$product_id is not used.

$price = ($price+$largo;
has an opening bracket but not a closing one.

global $post, $blog_id;
$post and $blog_id are not used.

So that leaves only:

function return_custom_price( $price, $product ) {
    $largo = $_POST['largo'] ;
    $price = $price + $largo;
    return $price;
}
add_filter( 'woocommerce_get_price', 'return_custom_price', 10, 2 );

add_filter('woocommerce_before_add_to_cart_button', 'medirlargo', 5 );
function medirlargo(){
    $largo = isset( $_POST['largo'] ) ? $_POST['largo'] : '';
    echo '<div class="custom-text text">';
    echo '<label>Largo (mm)</label>';
    echo '<input type="number" id="largo" name="largo" min="100" max="2500" value="'.$largo.'">';
    echo '</div>';
}

// Get custom field value, calculate new item price, save it as custom cart item data
add_filter( 'woocommerce_product_get_price', 'return_custom_price', 10, 2 );
function return_custom_price( $price, $product ) {
    if( isset( $_POST['largo'] ) ) {
      $largo =  $_POST['largo'] ;
      $price = $price + $largo;
    }
    return $price;
    // can't add to cart item data because its not a cart item at this point
}

This will alter the pice on the product page, but it won’t alter the price of the item when its added to the cart a few moments later.

To do that would need a lot more code than you would normally get in a forum answer.

You’ll need to look at product add-on plugins. Unfortunately, I think the free ones won’t do this. You’ll need to carefully review the premium product add-on plugins to make sure they will do what you want. You can ask pre-sales questions if you want.