not sure but theres an easy way to find out. get yourself an image of a grid or something youd easily be able to tell where it cuts off, then figure out the shape youd have to make it. also size itself doesnt matter its the aspect ratio(the height and width as compared to eachother)
it has the ability to scale down even the largest of images though that will significantly lag your phone if you use a huge one. but if the image is a perfect square its gonna take off the top and bottom to make it wide screen basically. hell heres an idea, get a picture of a ruler lol. paste it over an image that would be cropped, then look to see how bad it crops it, crop the picture itself to where the phone had it chopped off then use that as a template for future backgrounds