I also had this problem but now it is fixed for me.
When I contacted Clarence he explained that the skin uses high level graphics and the way MusicBee is set up it can have this problem on larger resolution screens especially is you scale over 100% as Steven said. I believe the only way to solve this problem at source is for the graphics handling of MusicBee to be rewritten to account for Hi-Def graphics but that would be a major undertaking.
In the meantime, Clarence suggested I refer to this post which can help with the problem for a lot of users. https://getmusicbee.com/forum/index.php?topic=31199.0
It works for me. Just remember that when Windows updates you may need to reset it again (not always tho).