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If you use the iFrame embed method then it takes automatically the 100% as width, and the width will be also affected by the element where the form is being embedded on.
Here's a sample of iFrame code:
The same happens when the form is embedded with the default method, it uses the 100% of the space where it is being embedded.
What I would suggest you is to check the element where you're embedding your form, increase its width so the form's width increases too.
Now, if you need to increase the form's width even when it displays on the entire screen, then you can do it by going to the designer section, styles tab and increasing the form width there:
Hope this helps.