I think the length and the story-telling were okay. Maybe a bit confusing at times but nothing that wasn't explained in the end.
About Aya, I think the author was hinting towards astral projection (that's just my hypothesis though).
If the length were to be increased, I think it would've had a less satisfactory ending. You can go on forever and ever on the topic of life and death, which is why this modest size seemed good enough to me.
The characters were simple, yet still had space for development and maturing. The background and artwork was amazing, I don't need to say anything here. The premise and the story itself was thought-provoking, something that's an important factor when distinguishing between a good and a sub-par psychological manga.
9/10 for me.