Scaling for Different iPhone Screens in Sprite Kit - objective-c

Question

I am creating an iPhone game in sprite kit. After weeks of research, I am still having trouble understanding how to properly size and implement sprites for each screen size.
I understand that these suffixes determine which image to use (depending on the aspect ratio of the screen)
#2x - 4s,5,6
#3x - 6+
I have read and tooled with different scaling modes in my view controller but had no luck and difficulty understanding them.
If I provide a background of 750x1136 (pixels) as the #2x, it will perfectly fit the iphone 6 but will be too big for the iphone 5. If scaling is the answer, how would "sprite kit" know I provided an image for the iphone 5 that I want scaled up for the 6, or vice versa? Is this a build setting? Same for characters, I need iphone 6 sprites to be proportionally bigger than the iphone 5 sprites.
How would I most appropriately size and scale sprites for the different devices? (easier to discuss in terms of the backgrounds that should be the exact size of the screen)
I am expecting to create one set of sprites for each aspect ratio using the resolution of the biggest screen size. Ex. #2x designed for iPhone 6 and scaled down for the 5 and 4s.

Answer 1

The 3x, 2x and normal images are not really intended to be manipulated that way. The three images should be essentially the same image with the 3x having exactly 3 times the pixel dimensions of the normal, the 2x having double dimensions etc.
If you need to scale the scene to better fit the format of a particular device, you may need to scale that when you create the scene, the way Apple sample code does:
var viewSize = self.view.bounds.size
// On iPhone/iPod touch we want to see a similar amount of the scene as on iPad.
// So, we set the size of the scene to be double the size of the view, which is
// the whole screen, 3.5- or 4- inch. This effectively scales the scene to 50%.
if UIDevice.currentDevice().userInterfaceIdiom == .Phone {
viewSize.height *= 2
viewSize.width *= 2
}