If you open an overlay with [up-size="grow"] it will be as wide as the contained elements. This usually requires the overlay elements to have a width or max-width, otherwise children with 100% width (or long paragraphs of texts) will cause the overlay to grow to the maximum width.

There's no public API to directly change the width of an overlay after it has opened. However, right now the size is only an attribute on the overlay container, and the width is styled using CSS. You can probably hack it by changing the attribute.

E.g. this is your response that wants a "wide" layout:

<form needs-overlay-size="large">
  ...
</form>

Here is a compiler that sets the [size]-Attribut:

up.compiler('[needs-overlay-size]', (element) => {
  let size = element.getAttribute('needs-overlay-size')
  let layer = up.element.get(element)
  layer.element.setAttribute('size', size)
})

Untested!