Problem was/is I'm using the german fritzing setup. While in the english one it is correct called "set board size" in the german one it is just called "Leiterplattengröße" (board size) the "set" is missing For dummies like me that makes a difference. May be one can add the word "setzen" (set) in a future german version.
Thanks for explanation
btw: in many programs I'm aware of setting/changing a value require just to hit "return". Thats what I've tried