What window size for landscape mode on mobile

I'm nearing the end to my game but I need help with setting the resolution.All this time i had the layout/window and margins at the same size 1280x720 now I dont know if this is the correct size because I'm trying to set It in landscape mode for android devices.

PS:Dont link me those tutorials,I've red them already but I got no clue.

Phones are all different. That's what the tutorial is talking about. Some phones allow a 1280x720, others will form a big black bar at the bottom. If you want to fit all phones, you need to ensure each level has enough extra padding to ensure it'll appear.

I assume you've been testing your nearly completed game on mobile devices frequently to ensure it's working.

Play around with the "Fullscreen in browser" setting.