[Brian]

Bob, I'm not 100% confident I understand your question, so please forgive me if I the answer doesn't help. :-)

OmniGraffle does support document scales; there are a fair number of folks that do 2d space planning with the app.

If you're looking for 3d space planning, though, it might be possible through some sort of hoop-jumping, but I don't know of anyone that does this, and it's not something built into the app AFAIK.