Click to enlarge:
Playing around with various reconstructions I think about 18 inches or 50cm, or maybe a bit less.
It would have looked like quite a narrow slot to observe the sun through.
The picture is a scaled composite of the Laser Scan of Stone 56, My reconstruction of 55 taken from Cleal et al and the two parts rejoined together and the excavation diagram of Gowland showing the packing stones of 55 and the position of 56.
UPDATE - More from E H Stone in an earlier post - http://www.sarsen.org/2013/03/great-trilithon-from-stones-of.html