After doing a bit of research, I realize now that a 1 kilometer wide sea tree is definitely not the size I want it to be. That would make it unbelievably huge. I want them to be more along the width of an average skyscraper, or perhaps bigger.

Click image for larger version.  Name: tallest-skyscraper.jpg  Views: 74  Size: 35.7 KB  ID: 25771

(Image stolen fair and square from a google image search. I most certainly did NOT create that myself.)

What this means is that I need to make my sea trees about 10% of their current diameter. I may need to do similarly with the coral, if only so I can make the coral more sprawl like. I'm actually happy with the size of the floatstone islands... I intended them to be amazingly huge.