Help CenterThis is not possible to do once you have started a project.
You could save your current project and then import the pdf into your new project and then import that PDF into your new project. See this video if you want to see how. Keep in mind, when changing sizes, it may slightly skew things from the original as the ratio of the paper could be different. As well going from a smaller size to a larger size of paper may reduce slightly the quality of images.
Just take the time before creating a project and know the size you want to use. Or at least when creating your first page, think is this the right size you want? If not, then you are not to far into that project, you could select everything on that page, copy it. Create a new project and paste what you copied into it. Yes you will have to make some adjustments. But it is better than having a whole book done, and then having to create a new project.