My goal is to create a UICollectionView with a layout as depicted below.

Use UICollectionViewDelegateFlowLayout method.
func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
let width : CGFloat
let height : CGFloat
if indexPath.item == 0 {
width = 100
height = 100
} else {
width = 50
height = 50
}
return CGSizeMake(width, height)
}
many ways to do this below links.
Without creating a custom layout of collection view it is not possible to achieve what you want.
You can create custom cell size using its delegate method sizeforitematindexpath.
Also, check this link which helps to create a custom layout.