Thank you for your quick response.
No, the resize box in the dialog box shown above is not checked.
Does this dialog box apply only when exporting a file in the catalog, or also when exporting a rendered slideshow?
Here is some more information:
My collection is a combination of high quality scanned images, cropped images, scanned lower resolution photos. The image pixel sizes are different for these categories. When I look at this collection in the library module and slideshow module, they are all identical sizes. When I create a slideshow preview, the also are identical sizes. Only when I export the slideshow do I see the images in different sizes. Can I force lightroom to render the jpeg slideshow so that the images all fit within the screen guides regardless of what each image pixel size is?
One more piece of information:
I had previously created a PDF Slideshow of the same collection in Photoshop Elements and all pictures were sized correctly within the top/bottom and side/side guides. I use Lightroom now, because I want the slideshow to be JPEG with captions, and Elements does not have that option.