The Combo Box widget in Mendix doesn’t support multi-selection with persistent associations out of the box. It only binds to one attribute (enumeration) or a single reference not a list of references.So when you select multiple items, the UI shows them temporarily, but they’re never written back to your parent entity.
1. Use a Reference Set Selector or List View Selector Instead
2.If your UI requirement forces a combo box style, use the Multi Select Dropdown widget from the Mendix Marketplace instead.
Assuming the sizing entity can be reused by other objects as well, you should change to a * - * relationship owned by the object that you are currently editing. Otherwise the size object can only be used by a single object.
However if that is what you need, then you'd need to retrieve the associated sizing objects and commit those separately, since they own the association.