How to scale a radius to fit in all devices?

Question

I am makeing a game in with a ball follows a path, actually is just an illution because the scene is the one that rotates. The problem is that in some devices the ball get out of the screen and in others the radius looks very small. I have tryed to make the radius equal (orbita.size.width / 2) but it doesnt work. (orbita is the orbit that the ball follows)

  class GameScene: SKScene {

   let sprite = SKSpriteNode(imageNamed: "circulo")
var rotation:CGFloat = CGFloat(M_PI)
let radius:CGFloat = 168


override func didMoveToView(view: SKView) {
    /* Setup your scene here */

    scaleMode = .ResizeFill
    node.position = view.center
    // 3) Add the container to the scene
    addChild(node)
    // 4) Set the sprite's x position
    sprite.position = CGPointMake(radius, 0)
    // 5) Add the sprite to the container
    node.addChild(sprite)
    // 6) Rotate the container
   rotate()
    sprite.color = UIColor.whiteColor()
    sprite.colorBlendFactor = 1.0
    sprite.zPosition = 4.0



    orbita = SKSpriteNode(imageNamed: "orbita")
    let padding2:CGFloat =  32.0
    orbita.size = CGSize(width:view.frame.size.width - padding2 , height: view.frame.size.width - padding2)
    orbita.color = UIColor.whiteColor()
    orbita.colorBlendFactor = 1
     orbita.alpha = 1
   orbita.position = view.center
    self.addChild(orbita)
    orbita.zPosition = 3
}

Answer 1

If your orbita.size is (it seems a circular orbita by "circulo" name):

orbita.size = CGSize(width:view.frame.size.width - padding2 , height: view.frame.size.width - padding2)

your radius would be:

let radius:CGFloat = (view.frame.size.width - padding2)/2