[Therion] point:passage-height

I'm using point passage-height to show the depth of some cobble banks. The map scale is 1:2000. The symbol almost fills the passage which is 9m wide and I can't find a way to scale it down. Using -scale xs makes no difference and I can't find any definition for the point or any metapost code to change it. I tried the layoutScales from Bruce but that made no difference. Does anybody know how I can modify it? Dave Clucas daveclucas.com mycaves.org.