HTML not resizing

I’m sorry I don’t have enough recommendation to comment your question and ask for more information.

To answer your question as I understand it :
This is CSS related.

If you only used the HTML part of your “3rd party site”, it is normal you don’t have any CSS style applied (unless it is inline CSS, but it’s not the case here).

I recommend taking basic CSS lessons before anything else.

What you need is to style your <select> element, as in this exemple :

select.your-select-class{
    width : 8O%;
}

I don’t know what you want your select to look like, so my answer can’t be more specific.