Try it with size set to 0.1. In screen mode, this becomes an aspect-ratio percentage (kind of) so your screen mode billboard is currently 2000x the size of your screen.
Ideally, create you own curve in the Content Browser → it’s under miscellaneous, afair. It may not be the best idea to modify the engine’s one as it may be used for something else.
When it comes to accessing assets, you can double click them or find them by clicking the tiny magnifying glass icon next to it, it will take you to the right folder. You can always make a copy, drop it in your project folder and work with that.
Why did I set the curve Key values of ‘8000,0’, ‘3000,1’ to no effect?