I think there are quite a few stone circles with larger diameters than Stonehenge.
Avebury comes to mind. What makes Stonehenge unique is the size of the stones in the circle.
ML/NJ
What also makes Stonehenge special, though not unique (there are other representations in different sizes and media around the world), is its almost exact replication of the discharge pattern of a high-current zeta-pinch aurora. In addition, there are various artifacts and petroglyphs numbering in the millions around the world that reproduce a wide variety of visual instabilities manifested by plasma instabilities. See Characteristics for the Occurrence of a High-Current Z-Pinch Aurora as Recorded in Antiquity,
IEEE Transactions on Plasma Science, Vol 31, No 6, December, 2003, p. 1212. There's a download link to this and other related papers at
http://public.lanl.gov/alp/plasma/papers.html.
Here is the abstract of this paper. There are other, more recent ones, with a lot more information.
Abstract—The discovery that objects from the Neolithic or Early Bronze Age carry patterns associated with high-current Z-pinches provides a possible insight into the origin and meaning of these ancient symbols produced by man. This paper directly compares the graphical and radiation data from high-current Z-pinches to these patterns. The paper focuses primarily, but not exclusively, on petroglyphs. It is found that a great many archaic petroglyphs can be classified according to plasma stability and instability data. As the same morphological types are found worldwide, the comparisons suggest the occurrence of an intense aurora, as might be produced if the solar wind had increased between one and two orders of magnitude, millennia ago.
Index Terms—Aurora, high-energy-density plasma, magnetohydrodynamics (MHD) instabilities, petroglyphs, pictographs, stonehenge, Z-pinch.