I don't think that's a good way to approximate size in this picture. You'd be better off taking the leaf in the foreground as baseline ; should be at most the length of a handspan.
This is actually one of the more benevolent deibus noraaki has drawn. Note that the marisa has at least some crumbs to eat, unlike other pictures where the marisa has been eaten, crushed, or totally neglected.
As for how daddy marisa has been cut clean in half I have no ideas... Gray Fox?
I think moeman's right. My guess is that Marisa did the "please eat up" thing, but Reimu is a Deibu and chose to neglect ko-marisa.
(In the fanon, some have yukkuri capable of splitting themselves as a sacrifice to feed their children. I think it's generally referred to as "please eat up", or something like that)