In the slide show editor, select a slide and then click on the caption text in the slide. On the right, you should see options to change the text formatting including size. You can also drag the text box to a new location on the slide.
I am not aware of a global setting to change all the captions. Currently, the default size is variable, depending on the length of the text.
Hope that helps
Thanks, Brian, that was great and it worked just as you said.
If anyone else can suggest a global fix I would be most grateful as I have hundred of photos and it is going to take me hours to reduce each one to 14 points.