How can I determine the size of individual slides in a PowerPoint slide deck?
There is a built-in way to measure slides! Publish all slides and use Windows Explorer.
Publishing all slides to an empty folder creates as many presentations as slides you have. Each new presentation stands for one slide and its Windows Explorer file size shows you which slide has the biggest pictures/audio/video.
File, Save & Send, Publish Slides, Publish Slides (button)
Select All - I recommend using Select All the first time so you can see the size of all your slides. Choose destination Windows folder. Publish
Review sizes and you can launch each individual-slide powerpoint file to see which one it is and whether you can reduce the size. After you have made ammendments you can individually choose which slides to Publish and see what effect your edits have made.
In my sample I had 3 slides with different kinds of media. And the first slide with 12 MB was indeed the biggest one.
This might depend on the specific version of PowerPoint, but in the version I'm using it is possible to save the presentation as 'Strict Open XML Presentation' file type, which is essentially a zip file with all the elements represented as files within the archive (in some cases this would actually be the default, so no need to 'save as'). So, one can rename the file from .pptx to .zip, open it, and see the sizes of the different element (as well as compression ratio). A good candidate place would be to look under ppt/media. Besides just large images (which can be then easily found and compressed or modified/removed within PowerPoint), in some cases it may contain images from unused master slides, and you may not even know they exist (especially if you are reusing presentations based on someone else's templates). In this case the way to handle it would be to open View -> Slide Master, and browse the master slides to find the one comprising the pictures (and potentially just delete it, if it is not used by any of the slides).
To make your slide deck smaller, quickly, try PowerPoint > File > Compress Pictures. This process just reduced a Mac PowerPoint file (version 16.11) that's photography-heavy from 40MB to 14MB with no degradation in presentation quality.