The above image shows the structure of Midgar's central plate (as seen from the slums) and the rendition of it I'm working on. The enhanced blue spot on the small left image is actually train cars. Notice how the space between two columns is about two cars wide. ...and, it looks like there's 24 columns around the whole structure. Using that information, and my own passenger cars (scaled by oblivion's character height) I've arrived at an area for the central plate.
That's essentially a map of the mod. I can't take credit for the line drawing, but it's overlayed on the grid representing Midgar's world space. Each white outlined box is an Oblivion cell. The total area for Midgar proper will be 361 Oblivion game cells, or 13 miles. There'll be a lot of outlying terrain, too.
...nothing's set in stone, of course!