Реализация
let scW = UIScreen.main.bounds.width
let scH = UIScreen.main.bounds.height
func collectionView(_ collectionView: UICollectionView,
layout collectionViewLayout: UICollectionViewLayout,
sizeForItemAt indexPath: IndexPath) -> CGSize {
if collectionView == col1 {
return CGSize(width:<##>,height:<##>) // supply ratio of screen width/height
}
else ...
...
}