Slideshow in LR

Weird! And I do not have experience with 5K screens, and I think this might be related to the problem, so see if some Mac users chime in with answers. (Images may be displaying as 1:1 screen pixel dimensions!)

I see different sizes in your clips- The LrC 're-size' in export will only 'up-size' small photos if [Don't Enlarge] is not checked. Small images (eg. 900x400px) will remain at this size. How is your 'Export' option set?

Have you tried checking the [Zoom to Fill] option in the SS?

And also strange, you have [Show Guides] checked, but I do not see them in your screen-clips.

Persevere!

If you 'Exported' with the Resize option checked and entered W 1920px H 1080px, Then ALL the exported JPGs should be of a pixel size that 'FITS' within those size limits! No exported files should EXCEED these dimensions.

eg. Any large image file cropped to be 'Square' would export to JPG as 1080px x 1080px. A narrow horizontal panorama might export as 1920px x 600px.

Check your Export dialog for Resize. Check that you are using only the resized files in the Slideshow.

And yet, even files that are "actually bigger" (eg. any 'original' photos) should display in the Slideshow module OK. Lightroom automatically resizes them to fit within the 'Guides' that are all "set to zero".

Where are you viewing the slideshow to see the images "too small"?

What type of screen are you viewing on? HD, 4K?

Provide a screen-clip of your Slideshow module using the [Insert Photos] icon above your reply.

Provide a screen-clip of the "too small" appearance.

WobertC, Thanks for replying. Yes, I just confirmed that the jpgs are the expected size. Many are actually bigger at 2800x1867 & 3800x1900. The Guides are at zero.

Is this because I am using jpg files?

No. The slideshow module will work with any file type in the Catalog.

You exported at 1980x1080. Have you actually checked that the JPGs in the Catalog are, in fact, 1980x1080?

And- How are the Guides (photo cell boundaries) set in the Layout Panel?

A hint: You can check a box in the export dialog that automatically 'adds' the exported JPGs to the Catalog. No need to Import files!