Update size parameter to accept callable for callback.set_size()

[OneSizeFitsQuorum]

Allow 'size' parameter to be a callable that returns an int. So that we can set size properly here

[martindurant]

This is maybe OK, but I didn't understand the original issue.

Certainly, this PR should add documentation about how to use this change, and a test showing it in practice.

[OneSizeFitsQuorum]

Thanks a lot for reviewing this! Maybe we can change this line from callback.set_size(getattr(f1, "size", None)) to callback.set_size(getattr(f1, "size", None)()) in here.
or add a wrapper function to ensure the parameters passed to set_size is a int, then we do not need update set_size(). What's your opinion?