Set the estimatedItemSize property of the flow layout to a non-zero value.

// Inside the view controller that has controls a collectionView.
override func viewDidLoad() {
  super.viewDidLoad()
  // Do any additional setup after loading the view, typically from a nib.
  let flowLayout:UICollectionViewFlowLayout = collectionView.collectionViewLayout as! UICollectionViewFlowLayout
  flowLayout.estimatedItemSize = CGSizeMake(100, 30)
}

Use constraints on the collection view cell content view,
or override sizeThatFits:.

// Inside the custom collection view cell class, override |sizeThatFits:|.
override func sizeThatFits(size: CGSize) -> CGSize {
  return CGSizeMake(*customWidth*, *customHeight*)
}