How do I crop a full-hd image to a resolution that doesn’t fit on the device?

At first sorry for my vague question I’m going to make it more clear.

The final result I want to achieve is sharing (using ngCordova social sharing plugin) an image that has a resolution of 851 x 315 (facebook cover image).

  • I shoot a photo through my Samsung Galaxy S4 (the resolution of the image is/will be 1080 x 1920)

  • I accept the image as you need to do that on android (the resolution of the image still is 1080 x 1920)

The cropper doesn’t have the set dimensions because it can’t, because the device resolution is too small.
How could I crop a full-hd picture crop it to 851 x 315 and still be able to work with the picture.

To show what the desired result will be I tried the same in Twitter (for iPhone tough), here I’m going to set the image as a new header image that’s (1500 x 500).