Running Mac with Sierra on both an iMac and a MacBook Pro. Adobe CC 2015.8. Cannot change the thumbnail size because the arrow that should appear when hovering over the edge of the timeline thumbnail does not appear. The thumbnails in the grid view can be changed with the slider but the timeline thumbnails size cannot be changed.
You're talking about the Filmstrip, do you?
If restarting you Mac does not help, try resetting the preferences: https://www.lightroomqueen.com/articles-page/how-do-i-reset-lightrooms-preferences/
This bug has been acknowledged by Adobe, and there is another workaround that doesn't involve resetting preferences: Filmstrip is covering / overlapping filter- and breadcrumb bar | Photoshop Family Customer Community
Please add your me-too vote to that bug report to make it more likely Adobe will prioritize a fix.
The above link did the trick. Thanks!!