Thanks, folks!
Here's what I did: Since I need the 3D file, I have to work in 3D. I also have to use Microstation J, ver. 7.0something - company's decision. Going back and forth from 3D to 2D didn't have an acceptable reduction in file size (it went from 28 MB to 27.8 MB). Scaling everything to a single depth (.0001 or similar) worked in that all the elements were on the same depth, but again, no reduction in file size. Also, I compress the files on exit, too, and even that didn't reduce the file size.
What I'm drawing has a lot, a whole lot of elements: it's a cellular cofferdam, built from flat web sheet piles, with each cell being 36 ft in diameter and the whole thing is about 1200 ft long. Hence, lots of elements.
Now, two things did it for me. The first, was to simply control-select the monstosity and drop the complex elements. This reduced the file size all right, but it also took away my neat sheet pile cells courtesy Arbed Steel. The second, and what I've decided to use, is to go with using the big file as a reference file. It still takes a few seconds to draw the cofferdam, but saving the file takes no time at all. The main topo is at the same scale and georeferences as the cofferdam, so my reference file lines up perfectly with the main layout. (Note: fencing the reference file and copying into the main file had the effect of boosting file size by, argh!, 27 MB!)
I'm not necessarily please with this, but it works and is easy to follow.
As a further note, Arbed has sheet pile .dxf and .dwg files available for download on their web site (can't remember the url right now, sorry!), which is great - I think. Each little sheet pile section comprises something like 25 line segments. Looks good, though.
Thanks again!