If you happen to have a High DPI monitor and the game's ui is showing up really small try the following:
Go to your Java Bin folder:
Here is location of where my version is currently at - C:\Program Files\Java\jre1.8.0_144\bin
Right Click on java.exe, then click to properties, go to compatibility tab and click "Override high DPI scaling behavior" switch it to Scaling performed by: System
Apply...
Next time your start up the client you should see the difference right away.