Paperform doesn't provide tools to modify the width of your embedded form.
If you are looking to change the size of your form's width, an alternative to the built-in tool is to apply some CSS rules within the page your pop-up form appears on. Providing the pop-up container with less available space may work to reduce the width of the overall form, although this isn't guaranteed.
Example CSS
The following example restricts the pop-up form container to a width of 400px. The example CSS rule should be added to the stylesheet for the page your form is embedded on.