Help. I don't want to make application and graphics twice as big as needed.

I'm very confused and I need some explanation. I mentioned something and now I have a problem.
First of all I only want the game for my phone(screen diagonal: 8", resolution: 854 x 480 pixels).
If I create an image in Gimp/Photoshop with the size of my phone's resolution(854 x 480 px) and open it with an image viewer(gallery) on my phone the image is way to small(because 8" is not 854 x 480 px).
But if the image is 427 x 240 px(half of my phone's resolution: 854 x 480 px) it fills the screen size of 8" in the image viewer perfectly.
Unfortunately the application on the phone is the exact opposite:
If the Fusion application size is 427 x 240 px it looks on the phone, like 854 x 480 px in the image viewer(it doesn't fill the screen).
If the Fusion application is 854 x 480 px it looks on the phone, like 427 x 240 px in the image viewer(application uses the whole screen size perfectly).
And that is bad, because now I have to work in Fusion with an application size of 854 x 480 and graphics that are twice as big as needed.
Is there something I can do as workaround?

Nevermind. I accidently used Adjust window size. Fit inside was the right option.