How is inline stretch size constraint determined?

How is the inline stretch size constraint (ie. the target width for a horizontally stretchy operator) determined? At https://w3c.github.io/mathml-core/#dfn-inline-stretch-size-constraint, we say "During box layout, optional inline stretch size constraint and block stretch size constraint parameters may be used on embellished operators.". However, I can not find where it says how this parameter is determined, or if (& how) we can affect it. (There is an open issue #64 that suggests minsize/maxsize might be applicable, but resolution has been deferred).

In experiments, I can only see horizontal stretch happening within mover (& friends), but not in mpadded or mtable, for example.
The target application is commutative diagrams, which will appear within an mtable. It would be acceptable to specify an explicit length for the arrows, if they could stretch at all; but it seems perverse to wrap arrows with an extra mover and empty (but wide!) script just to get the arrow length.

Read the MathML Core definitions for “inline stretch size constraint” and “embellished operator,” then review related issue #64. Compare the specification with the reported behavior in mover, mpadded, and mtable. Done means documenting how the constraint is determined, whether authors can affect it, and how the commutative-diagram case should be handled.