Alter lazyplay.sexp to tune the paths to your computer
Configure lazyplay.sexp with the file extensions you want and the arguments you want
Then move lazyplay.sexp to /etc/lazyplay.sexp (make sure it is readable by your user)
Change the arguments for mplayer between files via the terminal input (this will be started when you run lazyplay.rkt)
e.g.

> -fs
>

This will make mplayer use the fullscreen argument on the rest of the files
If you want to change it back to using no arguments, then just hit enter again, or enter a new set of arguments