[moriador]...

To be accurate, wouldn't the planets have to be truly tiny (as in almost invisible) -- or the entire contraption truly enormous? ;)

Actually, not really. :)

The orrey gives the planet's relative motions. It's not meant to be a "to scale" replica of the solar system, but one that yields useful information. Of course, if you wanted a completely accurate orrey, it being to scale would be necessary. :)