set a console variable

I read somewhere that setting r.Shadow.RadiusThreshold may help me get shadows on tiny objects. But how exactly do I set that? It sounds like a console variable, but have not seen or used a console before. Does it work in PIE? Please advise on how I can set this…

If you press the tilde (~) key while playing, that opens the console. If the console is open you can just type the command and hit enter, while typing it will even give suggestions based on what you’ve typed.