Silbury appears pretty small from Windmill Hill even on a clear day - it's a good view though, taking in the whole of the east-south-west skyline. A much closer view of roughly the same skyline is from the bus lay-by between Avebury and the Beckhampton Roundabout. There's also another good view but with a more westerly aspect from the top of Waden Hill looking onto Silbury and West Kennet.

If you've got a car you could do all three views starting with Windmill Hill, the lay-by and Waden Hill in a shot space of time. In fact, if you did it in that order (and I'm not too good at calculating this) but you might just be tracking the moon as it moves from south to west - someone else might be able to confirm that.