Gets the default dimensions that a new page will have when it is added to a design. It is possible for a user to resize a page without resizing the entire design, e.g. by clicking "Expand to Whiteboard". However, there will always be a single set of default dimensions for a design that is applied whenever a new page is created.
Returns undefined if the design is unbounded (e.g. Whiteboard or Doc).
Returns
A Promise that resolves with either undefined or the following object: