I'm afraid that's not possible. The page dimensions need to be consistent throughout the entire website project.

The system isn't really intended to have long pages that exceed one screen anyway. The different page dimensions are basically just to customize your project for different screen resolutions, including very high ones. Generally, this type of websites aims to contain all content in one screen. If you have more extensive text content, make use of the scrollable text field items that you can embed into the page.