Display just two columns, with multiple rows in a CollectionView using storyboard

You need not check the device size because we can use the collectionView width to calculate the width of the cell. Using the cell width you can calculate the height as per your need.

One more thing: You need to use UICollectionViewDelegateFlowLayout & confirm the delegate & implement method below

  func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {

   let padding: CGFloat =  50
   let collectionViewSize = collectionView.frame.size.width - padding

   return CGSizeMake(collectionViewSize/2, collectionViewSize/2)

}

Update: Swift 4.0

  func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let padding: CGFloat =  50
        let collectionViewSize = collectionView.frame.size.width - padding

        return CGSize(width: collectionViewSize/2, height: collectionViewSize/2)
    }

Note: I have answered the question considering the cells are square sized

Xcode 8: Swift 3

I know this is an older question, but I found a few issues with accepted answer.

I had to use UICollectionViewDelegateFlowLayout which is not CollectionViewFlowLayout

Then

In it 'Padding' is of type Int and when you try to subtract it from the variable collectionViewSize it throws an error because they are different types. Very easy to fix however.

I just added : CGFloat to the line

Finally, while there is probably a better way to do it, I had to match the padding by adjusting the collectionView leading and trailing constraints.

So all in all this is what my code ended up looking like

extension LandingPageViewController: UICollectionViewDelegateFlowLayout {

  func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

    let padding: CGFloat = 25
    let collectionCellSize = collectionView.frame.size.width - padding

  return CGSize(width: collectionCellSize/2, height: collectionCellSize/2)

   }

}

- (CGSize)collectionView:(UICollectionView *)collectionView
              layout:(UICollectionViewLayout *)collectionViewLayout
sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
CGFloat padding = 50;
CGFloat cellSize = collectionView.frame.size.width - padding;
return CGSizeMake(cellSize / 2, cellSize / 2);
}