Android : Android: create activity not full screen with transparent background

I'd like to create an Activity smaller than full screen and having around it a transparent background.

Using re-size function I was able to create layer smaller than full screen, but I don't know how to create transparent background.

In my mind, if you tap on background nothing happen, but, in my case (simply re-sizing) activity, when you tap somewhere outside of activity, it loose the focus and disappear.

Any idea? Many thanks!