Help CenterThis is not possible to do once you have started a project. The size of paper changes, the placement of the margins on canvas changes, everything changes. So you must start a new 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? Most of all, your research on Amazon should tell you the size of book you are making as well in most cases. If you are not that deep into your 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.
A newer method of getting pages from one project to another is JSON, this means after you have imported by JSON method, you can still edit and such. But since the page size changes, then you will have to make adjustments on the page still. Please see this video for help on importing by JSON.