Secret to getting Self-Sizing-CollViewCells to work?

Does anyone know the secret to getting self-sizing UICollectionViewCell to work.
I have a cell template, build using stackViews, all wrapped in a UIView with constraints to the Cell object (IB cell size is 320 x 185).
I have used:
Code Block obj-c
NSCollectionLayoutSize* groupSize = [NSCollectionLayoutSize sizeWithWidthDimension:[NSCollectionLayoutDimension fractionalWidthDimension:0.80] heightDimension:[NSCollectionLayoutDimension estimatedDimension:100.0f]];

But all of the cells appear at size 300 x 185 on iPhoneX screen size. Despite some cell having shrunken text and others with extra space.
Secret to getting Self-Sizing-CollViewCells to work?
 
 
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