Thank you for your answer. IMHO custom Document Presets are little bit different thing then what I mean with "custom page sizes":
As I mentioned higher: when you are creating alternative layout, you can't select custom "document preset" but only from predefined or custom "page sizes".
So this is why I want to Sync this.
This is an important feature, I think not only for me but for all users who works with alternative layouts. I do not understand how it is that no one has raised this issue yet.