Well, according to Jane's quote from Burl, he sez 1700 ft is the max, but I assume that's just the size of the biggest 'henge' studied in his terms.
Do we really know the 'definition' & purpose of 'stone circle' much better or in more 'scientific' terms though really? Y'know - what have, say, Stonehenge & Appletreewick really got in common? Just a thought.