How do I make an app for different screen sizes?

I have an created an app but when I open it after I have compiled with Cocoon or Phonegap sometimes it show zoomed out with a part of a black background and sometimes zoomed totally in and I or the user is not able to see anything . How do I fix this?

The problem is the way I have built the app or the way I have compiled it ?

Thanks!

Please help , I have seen tutorials but they are about maintaining aspect ratio and that requires to build my game from the beggining . I want an option that can totally stretch my app sprites , texts and background , stretch it totally . Please help , sorry for being this impatient.

HELP!!!

Check this Manual entry, property Fullscreen in browser https://www.scirra.com/manual/66/projects

It doesn't work , I have tried every option at th Fullscreen in browser property. HELP! Help!!!!!

Help, sorry for being impatient , really sorry.

Come on , isn't here anyone that can give me an insight on how can I solve this big problem? Appreciate ya!