In theory you project them. 6x6 projectors for which you can get 645 slide mounts are readily available (look for a Rollei on eBay, expect to pay a couple hundred) but if you go to 6x7 that is not the case, as I found out to my chagrin on seeing the price of the Goetschmann.
I actually scan my slides so that everyone can see them (90% of my audience is via the internet), but I still want to build a projector someday. In the mean time, I've largely moved to C41 so that I can wet-print. Actually I have a triptych on my kitchen wall that's from scanned slides: three vertical panels of 20x40" inkjet/canvas. The scans from 6x7 give as much resolution as a metre of canvas can hold.