@Mikael Cedergren -  I too cannot freely resize the phone wireframe library object. I can’t find specific mention in the Wireframe Library help article of the phone/tablet wireframe library objects having a set aspect ratio, but I suspect this is expected behavior either by design or due to a technical constraint. 

If this is something you would like to see changed, I’d recommend adding this as an Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know. If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.