There is no such specific size which you could export to.
You can try to export to the original size and keep resolution high so that the images do not get pixelated when you scale them.
What does "wrong size" mean? What about it is wrong? Describe in detail
The person creating the powerpoint presentation should tell you what size they need.
Usually 1000 pixels on the long side should be more than plenty.
I would use the projector's native resolution values with 'Resize to Fit' and 'Width & Height.' This will size landscape images to fit the screen width and portrait images to the screen height.
The below example is for an XGA 1024 x 768 projector. Keep in mind that if the PowerPoint is to also be used for viewing on a monitor with higher resolution (most are!) then the images will appear soft due to upscaling. To cover both usages determine the highest resolution monitor to be used and input those numbers in the LR Export module W and H boxes. The lower resolution projector images will look fine (down-scaled) as well as on the higher resolution monitor (no scaling).
(Corrected Resolution Values in example)
Thank you one-and-all!
The presenter was of little help to us.
Originally, I saved the pictures as "for email> he said the images pixelated. I then used the original file size and he said that the images sized like a thumbnail photo on the monitor.
I will need to obtain the ascertain the monitors resolution and try again. I know that once I get it corrected for presentation, more people will be able so properly use the PowerPoint with personal photos.
I will share the outcome as we go forward.
Again, Thank You everyone!