Hi guys!
I'm the author of TerraBuilder FSX, fire away with the questions, criticism, wishlists...

To answer some:
Yes, it is designed for smaller chunks, up to 2880x2880 pixels. The rationale is, it is easier to work on smaller individual chunks than the large ones, especially when creating masks. Also - smaller chunks are not neccessary to be re-projected because they are more granular.
Smaller chunks are also extremely handy if you are designing an airport and wanting to retain the default runways - smaller chunks can be precisely georeferenced/aligned with the default runway.
It definitely handles any and all temporal versions (seasons) - actually, most of the relevant INF file options are covered.
Big plus is that it loads and saves directly in the SDK INF format. You can also load in multiple, adjacent INF projects, or patition a large project into smaller INF chunks.
In the future, we intend to support small-scale terrain meshes, ideal for making sloped runways or small localized terrain corrections.
Cheers,
Misho