Yeah, I believe that 1 square is 5x5 tiles on the map...however, some buildings aren't quite aligned properly, or take up more titles when placed than their physical size. (for instance, our Small Neutral HQ is really 10x15 tiles, despite being 3x3 squares) Some alignment issues on the Notum Mine building cause it to overlap into the "border" space by 1 tile and only extend 4 tiles in the inner space on our land as well.
Also, there are some bugs with mesh overlap even if tenichally the tiles have space... for instance, we have a 2x3 square spot, with a 2x2 cafe in the south half. We placed a 2 tile object (bar whompah) in the NE corner, followed by a 1 tile gap, followed by another 2 tile object below it. Technically it "fits", but the lower of the two objects floats about 15m in the air at the same height as the little roof extension on the edge of the Cafe.

-Jayde