Android Question FCM Notification - wich graphic format/resolution for small icon on top left position?

I try to follow the Push Notification tutorial/example Push Notification
On B4a ide, when you choose Project / Select Icon you can modify the default B4x icon viewable on top left corner of app form
But for notification message, there is also a very small icon that appear on top side of Android system to indicate a notification is received
This icon is selectable with

....
n.icon = "Very_Small_Icon"
n.SetInfo(Message.GetData.Get("title"), Message.GetData.Get("body"), Main)
n.Notify(1)
....

Wich graphic format/size/resolution must have?
There are web site to obtain this little icon from main app icon?