To change the size of your document, make sure nothing is selected in Lucidpress. Then, on the right panel, go to the "Size and Position" section. From the dropdown, you can select the desired paper size. This will cause your canvas/document to resize, however, your content will remain in its original position.
After you do this step, click View > Show off page content so that you can you can see all of your content off of the canvas area. Highlight all of your content and then drag the box from the corner to change the sizing of everything to scale down (or up) to your desired size.
Select areas of text that disappeared or have the red + box and change the font size to adjust to your new document size.