I had the same issue. Tried uninstalling & installing again. Nothing worked out.
Then instead of getting MPlayer X from App Store, searched from their site and downloaded & installed the player. Basically, copied downloded 'MPlayerX.app' in 'Applications' folder and replaced with existing one.
Now, fullscreen works without any issue.
One thing to note is, the downloaded player's version is 1.0.22.1 whereas the one from App Store had 1.0.14.
Also, the new player has 'Check For Updates' menu as well.
What MPlayerX Can & Can’t Provide. Before we move forward, you can refer to the following pros. Mac cleaner can help Mac users easily and quickly remove junk files and unwanted files from Mac computers. This year, Apple releases new version of macOS – macOS High Sierra 10.13. In this article, we would like to introduce you the best 5 Mac cleaner for macOS High Sierra.
Mplayer Osx
Just in case if you need, here is the download link