I figured it out. I had tried editing that input field and under width it said 100 px. So I deleted that (leave it blank for 100%, it said) and saved it but that didn’t change anything on the page.
Then when I went back to the form editor and clicked to edit that input field, the 100 px was back. So this time I entered 300, saved and my form looks right now.
Not sure why leaving that field blank didn’t work to enlarge the fields, but there you have it.