How to resize the collection view cells according to device screen size?

Swift 4

Multiply the collectionView's frame width/height in an implementation of the sizeForItemAt method.

class yourClassName: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {


    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
            return CGSize(width: self.myCollectionView.frame.height / 6 * 5, height: self.myCollectionView.frame.height / 6 * 5)

        }


}

self.myCollectionView is the reference from you collection view Item
it's important to implement UICollectionViewDelegateFlowLayout

And because it will not fit another cell you will have only one row.

Implement sizeForItemAt method according to the view's frame size

Swift

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

    let height = view.frame.size.height
    let width = view.frame.size.width
    // in case you you want the cell to be 40% of your controllers view
    return CGSize(width: width * 0.4, height: height * 0.4)
}

Objective C

- (CGSize)collectionView:(UICollectionView *)collectionView 
              layout:(UICollectionViewLayout *)collectionViewLayout 
   sizeForItemAtIndexPath:(NSIndexPath *)indexPath {

   CGFloat height = self.view.frame.size.height;
   CGFloat width  = self.view.frame.size.width;
   // in case you you want the cell to be 40% of your controllers view
   return CGSizeMake(width * 0.4, height * 0.4);
 }