I just did one, and struggled to find the right way, but here’s what I did:
1) File → Export → Quicktime Movie (NOT using Quicktime Conversion)
2) I used custom settings, using the aspect ratio of the slideshow
3) Compressor → Photo-JPG @ 75%
4) Pixel Aspect Ratio → Square
Once that is exported, you will have a nice, big, hi-res file that looks great. I then exported out of Quicktime to the smaller size using the H.264 compression for web use.
Others might have better answers, but that worked great for me.