In current state, i think, we don't need this. Main purpose creation layout is just save a pattern in graphical format. Yes, user can't change base size of the paper sheet (A0), but if need more length you can extend the paper length. Custom size will be useful only if really need them. Valentina can't now print patterns, this mean user will use a external program for printing where they can change the size. Also patterns usually big and use even A1 size not enough in many cases.
For example if use svg. You can ungroup objects and change position how you like. And then print. My customer did this.
Yes, in current state, this feature not very useful, that's why i want to add automatic creation layout.