I have the file. It was so large because you did not Clear Generated Data on the mesh.
I have 15.0 on another computer, but I made these snapshots on a newer version. I can redo this on 15.0 if you need more help.
I suggest you idealize and simplify the problem somewhat and use a zero thickness surface for the "building". That is as simple as adding a MidSurface feature on the solid building. I also suppressed the extrusions that make up the block of air and the cut body operation. Instead I use an Enclosure. The solid and surface body are put into the same part using Form New Part.
Then you will have one solid with a zero thickness walled building to mesh.
The next thing I did was to create 5 planes, one on each face of the midsufaced building, and Slice the air up using those 5 planes. Select all the solid pieces and Form New Part to have the mesher connect them together. These solids are all put in a Multizone control to be meshed with hex elements. Here is the skewness report.
I have not added any mesh refinement yet, that still needs to be done to get many elements around the windows and over the building.
For other members, I attach an ANSYS 18.2 version of your file here, which you can't open. But if you want a 15.0 version, let me know.