In the Nekomonogatari-Black anime, yōtō "Kokoro-Watari" (妖刀「心渡」, sometimes translated as demon sword "Heartspan") is shown with different lengths, but it always appears to be bigger than Araragi. The size changing isn't very noticeable unless you are actively checking, so it could just be due to animator laziness, but the fact that it is longer than Araragi is seems deliberate as it is drawn that way consistently in every scene.
Here is one screenshot of the sword, compared to Araragi. Both red lines are the same length.
Even if he is leaning over in the picture, it's pretty clear that the sword is bigger than he is. It's also clear when he gets cut in half:
However, despite the fact that the sword is taller than he is, it fits into his body. It even fits in Shinobu's body, though of course that could just be one of her abilities. But to the best of my knowledge Araragi should not have any such ability. In addition, the sword is stuck in the ground after his legs disintegrate, which also should not be possible if it was inside of him.
This could very well just be SHAFT following their usual style with the Monogatari novel adaptations, where the visuals aren't meant to be taken literally. Or it could be an indication of another ability of the sword. Is there any evidence that yōtō "Kokoro-Watari" has some ability that would allow it to fit inside Araragi's body (e.g. changing size)?