Can I change my form's width when embedding it as a popup?

There is no official way to make the form area any smaller.

However, you might be able to use some CSS to give the popup form less available space, which could work at reducing its width.

Example CSS

To restrict the width the popup uses to 400px, you may add this CSS to the page that the form is embedded on.

.Paperform__popupcontent {
    max-width: 400px;
}