When you create the embed code initially, it comes with a width="100%" property.
So if you have not edited that, then the iframe should expand to the full width of the container (whatever that might be) that you nested it under. So there’s a good chance the limitation you are bumping into has to do with the structure of your wordpress site.
If I just stick that embed code into a flat HTML document:
My grid view will expand to fill the entire width of my browser window:
So you need to look at the structure of your wordpress site, and see what you are placing the iframe inside of. Whatever the maximum width of that container is (it’s very likely a <div>), that will also be the maximum width of your embedded grid.