Whenever you publish a form on your website then it automatically takes its width depending upon the place it is added. If it is inserted in post/page then it takes the width of post container. If it is added as widget in sidebar then it adjusts its width to that of sidebar.
However if you want to specify the width then you need to add width parameter to form-wrapper ID. This can be easily accomplished by using Styles and Layouts for Gravity Forms plugin.
A little Brief of Styles and Layouts for Gravity Forms Plugin
This plugin is based on WordPress Customizer API that lets you instantly see the changes you make without any need to refresh the page. It has more than 100 settings to customize the look of your forms. It is fully responsive and you can set width/height in percentage as well as pixels.
How to use Styles and Layouts for Gravity Forms to Change Width
Once you activate and install this free plugin from WordPress plugin directory you will get a new panel in customizer. Initially you will only get the option to select the form. Once you select the desired form then Styles and Layouts plugin will automatically save your settings and refresh the page.

This time when you will navigate to Styles & Layouts Gravity Forms panel then you will notice that a lot of new sections are there. All these sections contains settings for particular part of form ex form wrapper, confirmation message, submit button etc.

Option for setting the width can be found under Form Wrapper section. You can give width in pixels, percentage or em. If you would like to have responsive design then width should be entered in percentage.

You can also do other settings like background colour/image or give margin and padding to your form from this section.
If you set the width by editing the style.css file of your theme then whenever you will change or update the theme then your styling will be lost. If you do styling using the plugin then the settings will work even when you change or update the theme.
Here is the video tutorial for this article
If you want to ask anything regarding this tutorial then drop a comment.

Hi, how can I add padding or margins between the radio buttons? My choices are all too close together at a desktop size. Also, I can’t get to the next page on my form because the “next” button is in edit mode and whenever I press on it, instead of going to the next page I’m forced to just edit it! Please help.