You need to do this programatically.

Implement UICollectionViewDelegateFlowLayout in your view controller and provide the size in collectionView:layout:sizeForItemAtIndexPath:

func collectionView(_ collectionView: UICollectionView,                        layout collectionViewLayout: UICollectionViewLayout,                        sizeForItemAt indexPath: IndexPath) -> CGSize {        let kWhateverHeightYouWant = 100        return CGSizeMake(collectionView.bounds.size.width, CGFloat(kWhateverHeightYouWant))}

You will also want to call collectionView.collectionViewLayout.invalidateLayout() inside your view controller's viewWillLayoutSubviews() so that when the main view's dimensions change (on rotation, for example), the collection view is re-laid out.

Swift 4 Update

func collectionView(_ collectionView: UICollectionView,                            layout collectionViewLayout: UICollectionViewLayout,                            sizeForItemAt indexPath: IndexPath) -> CGSize {            let kWhateverHeightYouWant = 100            return CGSize(width: collectionView.bounds.size.width, height: CGFloat(kWhateverHeightYouWant))}

Inside your view controller override viewDidLayoutSubviews method

@IBOutlet weak var collectionView: UICollectionView!override func viewDidLayoutSubviews() {    super.viewDidLayoutSubviews()    if let layout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout {        let itemWidth = view.bounds.width / 3.0        let itemHeight = layout.itemSize.height        layout.itemSize = CGSize(width: itemWidth, height: itemHeight)        layout.invalidateLayout()    }}

(collectionView property is your collectionView)

Use Following code for set the width of UICollectionViewCell.

func collectionView(_ collectionView: UICollectionView,                    layout collectionViewLayout: UICollectionViewLayout,                    sizeForItemAt indexPath: IndexPath) -> CGSize {            return CGSize(width: screenWidth/3, height: screenWidth/3);}