This is a picture from my cathedral model.
I build it in segments, so I can make it as long as I want.
The model is made with terrains and the size of the file for this picture is 81 MB.

Here you can see the light coming through the terrain made stained windows.

This is a later version of the model. In the first version I made the main columns with cylinders, here they are changed to terrains.
The result is a significant reduce of objects. I can handle the model much better now.

Here I placed second floor on the first one.

The light in this model is always a challenge.

This is a very early version. Here you can see that the model is build with segments.

The length of the cathedral model depends on the capacity of your memory (MB).