I don’t do wordpress so can’t help with any aspect of wordpress but if you post a link to the page I’ll see if there’s anything CSS can do to help with the issue.
I’m guessing the form code is embedded in an iframe and that means your options may be limited as you will have trouble in making an iframe responsive as that means the height would need to change also to fit the content length. You can probably change the iframe width to 100% but you will still have the form in a scroll box.
If your form is not in an iframe then it should be possible to adjust as required using the relevant CSS.