The Xenomorph prototype Bill Malone has is about the size of two basketballs, one on top of the other.  I don't know how that relates to the one they did use, but it was probably on the end of a rod or pipe that came out of the back; the prototype is just the front, sort of like a dish.  There are a couple of shots in the movie where the back of the Xenomorph is at least >suggested<--you see it laying down its pretty sparkly trail, with the hair hanging down to one side.