Mail Chimp for WP plugin input width help

Hi!

I have been looking for away to adjust the width of the input forms or overall form towards the bottom of my site: http://www.onepricedeliverydrycleaners.com

Generally, the input fields span the width of the entire page. This looks very odd at larger widths, and I want to constrain it. So, I’ve tried all the options from this post for correcting this issue:
https://wordpress.org/support/topic/plugin-mailchimp-list-subscribe-form-mailchimp-change-width?replies=33

I even tried changing the code:
#mc_signup_form {
width: 300px;
}

to:

#[mc4wp_form] {
width: 300px;
}

as that was what the form was called, or at least that was the short code for entering into text fields.

Any ideas why none of this worked?

Thank you!