Photo Manager has very limited editing functions for images and adjustable resizing is not one of them, however it is possible to resize images using one of two export presets as follows:
In the PM gallery view, right click the image you want to resize
Click Export, Export selected from the pop up menu
The dialog shown below will open ensure Export one by one is selected and click OK
In the next dialog - Select either of the options 1 or 2 - the export image size will be shown at 3 as shown below
Click OK to export
I would also suggest you select the Change filename option as well to avoid overwriting the original file and enter a new filename.
You can also change the location of where the file is stored in this dialog.