JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
This should be fixed by injecting this CSS code:
width: 100% !important;
This guide will help you to inject the code to your form: How-to-Inject-Custom-CSS-Codes
Take a look to my cloned form of yours where I applied this code and it is working fine: https://form.jotform.com/61398146268970.
Hope this helps.