The largest I have looked through is the 14 inch Celestron at Mapleton (thanks Paul) on a fairly ordinary Queensland night. Got a good view of part of the moon with a large Panoptic (34mm I think), I still remember the feeling as though I was going to fall into the eyepiece.
That's why I like to go out to Astroron's. On any given dark sky night there will always be a 16" there, but the chances of a 25" or a 30" are pretty good