I’m pretty sure it should be just an easy task, but how can I stretch the Menu background to the full screen size?
There should be no visible rest of the ingame scene but I cant find it…
thanks in advance.
You need to configure the Canvas Scaler of the Canvas. Below is an example of it being configured to stretch everything according to the height of the screen.
thats my problem, its already matching to the height, so everybody should easily see all buttons, but then the left and right part is missing and whatever I do they still miss (already played with all those scaling things)…
If I tell the reference resolution to have an Y of 720 its wider and decreasing the number widens it, but then the menu points vanish more and more…
I finally found the solution 
I had to strech the menu itself not only from top to bottom but to all sides, then there was a space of 193 pixels on both sides… just had to remove them and the menu stretches well to all sides 
[solved]