In 4.11, after setting Standalone Window Size to 1920x1080, it plays in correct, desired HD resolution. In 4.12, it ignores it when playing in ‘Standalone Game’ window (it opens is some other, smaller size). Only New Editor Window (PIE) respects the desired resolution.

Hello, I’ve reproduced your issue, and have entered a bug report, UE-32065. Thank you for your report. Have a great day