Those were the good old times, I remember looking for my Map Colors.txt file.

It was hard work to get colors in. When I was using Zap Placer, they hadn't invented overlapping yet. Or they didn't include it in the file

The editor must be a newer one, like X-tra Editor or StarForge.
The old ones aren't really editors, but mods. You can use Arsenal II to make the size of the unit like 1x1 pixel or something and run that as a modded editor.
You actually had to double click on "Starcraft Campaign Editor" to get to it. You actually could have picked "Starcraft" which I never tried because I use nocd hack, but it would probably have a game where all the units took up no space at all. Of course the building size was probably still there...