A size part on a packaging machine, for example, is a part that can be swapped with another, larger or smaller, one so the machine can accommodate products and their packaging of different sizes.

Our size parts also give your machines exceptional flexibility. Just imagine that, as standard, a single Norden line can handle tubes that range from 10 to 65 mm in diameter simply by swapping size parts.
https://www.nordenmachinery.com/sites/norden/files/component...

The particular opening (Patent Pending) permits to handle in absolute safety even the smaller carton with internal separator. The very good production characteristics, the electrical pitch closures and the whole of guides, references and adjustments made easier the size part change.
L'ouverture particulière (Patent Pending) permet également de manipuler en toute sécurité les étuis munis de séparateurs internes. Les caractéristiques optimales productives, les fermetures pas électroniques et l'ensemble de guides, les réfrences et les réglages, permettent d'effectuer un changement de format simple
https://pdf.directindustry.fr/pdf/axomatic/encartonneuse-hor...

I therefore suggest pièces de changement de format.

• Un fonctionnement polyvalent et flexible, sans pièce de changement de format de capsule
https://www.zalkincapping.com/fr/produits/visseuse-par-courr...
(good ol' André Zalkin who in 1961 moved his production plant from Paris to a village a little over half the size of mine barely 30km from here).
Comme sur toutes les têtes Zalkin, les changements de formatsont rapides et sans outils.
https://www.zalkincapping.com/fr/produits/tetes-de-bouchage-...

Zalkin appears to call these 'change parts' (cf. your subsequent comment):

They are simple, versatile and they accommodate cap sizes up to 135mm without the need for cap or container handling change parts.
https://www.zalkincapping.com/about/beltorque/