I've downloaded a .blend file recently and all of the interface elements look very small. I remember that there were an easy trick to make everything bigger (or smaller) when I was using Blender 2.49 long time ago. My question is how to do it increase or decrease the size of the buttons within the panels using User Preferences and without using it?
pressing Numpad + or Numpad - you can zoom in or out on any window (or control) in Blender.
To resize the panel drag it with the left mouse button.
As an alternative, if this is happening on a file you didn't create, you can preserve your own default layout, uncheck the LoadUI box when loading the file.