OK. Here is CC's picture with labels.
Taking Endoceras for our scale I can estimate apparent sizes. If this Endoceras is 13 feet long then it is about 2 feet in girth.
The "brain" corals look to be almost 4 feet in diameter. And the horn corals appear to have foot-long horns. The crinoids look to be between 3 and 4 feet tall. And the brachiopods (clams) seem to be about a foot in length.
In 35th scale that translates to 0.87cm for the horn coral, 3.5cm for the "brain" coral, 2.5 to 4cm for the sea weeds and crinoids. 1cm for the bivalves (this seems a bit large to me.)
But how big were horn corals???
"Some solitary rugosans reached nearly a meter in length. " http://en.wikipedia.org/wiki/Rugosa (That would be about 3cm in 35th scale.)
"The largest colonial rugose coral on the Indiana shore (directly below the Interpretive Center) is a Prismatophyllum colony 11 feet (3.3 m) across. A 30 foot (10 m) colony is reported on the Kentucky side!" http://www.fallsoftheohio.org/DevonianCorals.html (Hmmm. That would be 8.6 to 28.6cm across!)
GOOD NEWS! According to this article table corals existed! I don't have to remove what is there already.
According to http://en.wikipedia.org/wiki/Ordovician though solitary corals were common, reef colonies developed during the Ordovician. And there are images available through this same site of the types of algae that existed at the time.
Why didn't my earlier searches come up with this stuff ???
Oh well. Back to the Man Cave!