How to change UICollectionViewCell size programmatically in Swift?

Make sure you conform to UICollectionViewDelegateFlowLayout for your class (for swift4)

eg. class MyClass: UICollectionViewDelegateFlowLayout

Prior to Swift 3

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

    }
//Use for interspacing
    func collectionView(collectionView: UICollectionView,
        layout collectionViewLayout: UICollectionViewLayout,
        minimumInteritemSpacingForSectionAtIndex section: Int) -> CGFloat {
            return 1.0
    }

    func collectionView(collectionView: UICollectionView, layout
        collectionViewLayout: UICollectionViewLayout,
        minimumLineSpacingForSectionAtIndex section: Int) -> CGFloat {
            return 1.0
    }

// For swift 3

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

}

func collectionView(_ collectionView: UICollectionView,
                    layout collectionViewLayout: UICollectionViewLayout,
                    minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
    return 1.0
}

func collectionView(_ collectionView: UICollectionView, layout
    collectionViewLayout: UICollectionViewLayout,
                    minimumLineSpacingForSectionAt section: Int) -> CGFloat {
    return 1.0
}

// For swift 4

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

        }

        func collectionView(_ collectionView: UICollectionView,
                            layout collectionViewLayout: UICollectionViewLayout,
                            minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
            return 1.0
        }

        func collectionView(_ collectionView: UICollectionView, layout
            collectionViewLayout: UICollectionViewLayout,
                            minimumLineSpacingForSectionAt section: Int) -> CGFloat {
            return 1.0
        }
    }

use the same code as shown above for swift 3 but just make sure your class conforms to protocol UICollectionViewDelegateFlowLayout

Swift 3

Methods from @sumit-oberoi answer won't be called if you are using Swift 3. To make it work make this adjustments:

1. Conform to the UICollectionViewDelegateFlowLayout protocol.

2. Implement these methods:

   func collectionView(_ collectionView: UICollectionView,
                       layout collectionViewLayout: UICollectionViewLayout,
                       sizeForItemAt indexPath: IndexPath) -> CGSize {
   
   }
   
   func collectionView(_ collectionView: UICollectionView,
                       layout collectionViewLayout: UICollectionViewLayout,
                       minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
       return 1.0
   }
   
   func collectionView(_ collectionView: UICollectionView, layout
       collectionViewLayout: UICollectionViewLayout,
                       minimumLineSpacingForSectionAt section: Int) -> CGFloat {
       return 1.0
   }
   

Notice the changes:

1. add _ before collectionView in the method name
2. NSIndexPath changes to IndexPath

Swift 3

override func viewDidLoad() {
    super.viewDidLoad()

    let cellSize = CGSize(width:80 , height:80)

    let layout = UICollectionViewFlowLayout()
    layout.scrollDirection = .vertical //.horizontal
    layout.itemSize = cellSize
    layout.sectionInset = UIEdgeInsets(top: 1, left: 1, bottom: 1, right: 1)
    layout.minimumLineSpacing = 1.0
    layout.minimumInteritemSpacing = 1.0
    myCollectionView.setCollectionViewLayout(layout, animated: true)

    myCollectionView.reloadData()
}

if you use Gaurav Rami's code bellow

let cellSize = CGSize(width:80 , height:80)

let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .vertical //.horizontal
layout.itemSize = cellSize
layout.sectionInset = UIEdgeInsets(top: 1, left: 1, bottom: 1, right: 1)
layout.minimumLineSpacing = 1.0
layout.minimumInteritemSpacing = 1.0
myCollectionView.setCollectionViewLayout(layout, animated: true)

and get this message "Snapshotting a view that has not been rendered results in an empty snapshot. Ensure your view has been rendered at least once before snapshotting or snapshot after screen updates." you should remove animated when setting view layout for collection view

myCollectionView.setCollectionViewLayout(layout, animated: false)

then the message will disappear

Try to use UICollectionViewDelegateFlowLayout method. In Xcode 11 or later, you need to set Estimate Size to none from storyboard.

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: 
UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
  let padding: CGFloat =  170
  let collectionViewSize = advertCollectionView.frame.size.width - padding
  return CGSize(width: collectionViewSize/2, height: collectionViewSize/2)
}