I just purchased a new Surface Pro 4 about a week ago and have the same problem; the Windows Media Player controls for both audio and video playback are so small as to be useless. Also linked an icon in the Taskbar and that is also noticably smaller than the other icons. I also have gone through this discussion and see no solutions. Gonna take it back to the Microsoft store and see if they know what's what. Microsoft seems to have a history of "If it ain't broke, fix it 'till it is." By the way, I do not have this problem with my custom built desk computer, updated from WinXP to 7 to 10. And both machines tell me I'm running the latest version of WMP.
You can change the size of the icons with this article here. Also be sure to open the feedback hub in the app store and send in your problem. But yea, please report back, it'd be interesting to see what they have to say.