I was just wondering if anyone knows why these particular keyframes are longer than the others? I can see that the tips of the yellow lines for keyframe 0 and 16 are green, but I'm not sure why.
These are Grease Pencil keyframes that are created when drawing with the Grease Pencil tool.
You can change the Dope Sheet editor mode to Grease Pencil from the Dope Sheet header menu to view and/or delete these keyframes:
Alternatively, you can manage the keyframes from the Grease Pencil Layers panel in the 3D View Properties Shelf (N).