I'm designing for small desktop screens since it makes more sense to scale up than scale down. When sharing desktop prototypes with stakeholders with larger displays, they have two options: Show the prototype in a scaled down version (this reduces the prototype to smaller than the original design on 15" displays. Their other option is to go fullscreen, but going fullscreen fills up the entire screen and makes everything seem bigger than it is.
How about: An option to force the prototype to show in original size like the desktop app preview allows us to do. This would create empty space around the prototype but still allow it to show the sizes accurately.