UICollectionView Custom Cell to fill width In Swift

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);
}

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)

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))
}