You mean like this:
Steps to reproduce the bug:
1. down size REAPER horizontaly (to its smallest state)
2. down size the mixer (you'll notice it will let you go down below the actual minimum size!)
3. resize REAPER to normal size or even to fullscreen!
To get a even smaller mixer repeat steps 1 and 2 till the mixer has shrunk to your desired size.
NOTE: using the buttons on the MCP may get a bit hard.