There is no such option. Sad )
I don’t recall if that was intentional or not, but we’ll check it out. For now you could use calc to access viewport units.
Thanks for writing in! The way the content dimensions are setup to work for the header is that you can define a Length and a Max Length on the overall content, and then if you need to have a little bit of a spacing buffer beyond all of that, you can do so with the Outer Spacing feature. The Outer Spacing is intended to add a small amount of fixed spacing to the ends as you can use % values in the Length control to get essentially the same values as a vw or vh unit. Hopefully this helps to clear things up a bit!