A new beta is available for Chester One that (hopefully) addresses some full-screen scaling issues as well as adds in a new Borderless-Fullscreen mode.
If you're still having the issue where fullscreen mode is squished try out the Borderless mode to see if that solves the problem. (You may still have black bars on the top and bottom of your screen to keep everything at the correct aspect ratio. Those are normal for now!)
This patch is the first part of an HD-ifying set of updates that I'm working on, with the next adding in scaling options and removing the forced aspect ratio.
To enable to beta branch right click ChesterOne in your library->Preferences->Beta->'BETA' from the dropdown list.