My images are too small in slideshow mode and do not fill the screen. How do I enlarge them to fit the full screen. I have ticked zoom to fill frame with no luck. Currently the images are 720x480. Many thanks Serena
I think you may have to Export the images, setting them to re-size to a larger size (eg.1920x1080 pixels)
Export to the same folder, add to catalog, uncheck [Do Not Enlarge], Re-name is not necessary as Lr can append a "-2" creating a unique name.
Now re-select the enlarged photos for the slideshow.
Be aware, enlarging may not preserve any perceived sharpness in the smaller photos.