Resize form field

I’m having trouble resizing the form field so it fits in my sidebar. I’ve only been using CSS for a few months so it could be a lack of knowledge on my part, but I would really appreciate guidance on how to make this work!

The sidebar is only available on the internal and post pages- here is a link to one of them: https://therubylake.com/eventideas/

Thanks!

https://www.ads-software.com/extend/plugins/mailchimp-widget/