In newer versions of Minecraft: Java Edition, the ability was added to navigate around in-game menus using your keyboard by using the Tab key and pressing Enter to select the highlighted option. If you click on the GUI Scale setting to change your GUI size, it works as intended. However, using your keyboard instead will change the setting, but not update your GUI Scale.

 
Brief video of this (I uploaded it to YouTube because it's larger than 10MB, sorry!): https://www.youtube.com/watch?v=g-sBAM7De0A