I found it.

Export. Once you hit 'Export', you will be shown a 2-step export dialog.

On the first page, make your selections, likely the top option, and hit 'OK'.

On the second page, at the bottom, click on 'Advanced' and in the 'Advanced Settings', under 'Object Size' you can set the size to resample the photos for export.

Note: On page 2 of the dialog, you must set a destination folder in the textbox at the top of the dialog. If you want to 'Export' your resampled versions of the photos to a new folder that does not yet exist, you can just type the path to the new folder, including the new folder name, and Magix Photo Manager's export function will create the folder for you and place the resampled photos in it.