The issue is that either WooCommerce or your theme is overriding the iframe dimension.

• You can try using additional CSS and adding !important to override other styling (though you mentioned that you already tried CSS but I don't know what you tried);

.woocommerce-iframe
{
    width: 550px !important;
    height: 300px !important;
    max-width: 100%;
} /*replace the selector*/

• Add a wrapper div to the iframe and restrict its width;

<div class="youtube-embed-wrapper" style="width: 550px; max-width: 100%;">
    <iframe width="550" height="300" src="https://www.youtube.com/embed/XXXXX" frameborder="0"></iframe>
</div>

• Inspect the iframe with devtools for conflicting rules.

• You can also try disabling your plugins one by one and check for conflicts.