The helpful MAGIC site provides this information:
“The monument includes a henge monument set on a gentle south-east facing slope in an area of undulating chalk downland immediately east of the Wylye Valley. It comprises a central area between 30m and 33m across raised 1m above ground level. Surrounding this is a ditch, from which material was quarried during the construction of the monument, and an external bank. The ditch has been partly infilled over the years but survives to a depth of 0.5m and is c.10m across. The bank is 0.2m high and 10m wide. In the south-west portion of the ditch is a causeway c.4m across.
I must say I’m not particularly convinced by these figures but can’t do any better. I was preoccupied at the time by the sheer excitement of tracking it down and taking in the feel of its location and the hillside opposite, which you can see in the photos above.
I was sadly quite thrilled to see the henge show up on Multimap’s aerial photo.