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In order to widening the grid, please inject the CSS Code that follows:
width : 400px !important;
The following image shows the result:
You may copy the above mentioned CSS Code and inject at the end of the section, as show below:
Access your form in edit mode, click on Preferences, the Preferences screen will pop up, click on Form Styles tab, inject CSS Code at the end of the section Inject Custom CSS.
This guide will help you step by step on how to inject CSS Codes: http://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes.
I hope this helps.