I officially went insane last night and I need to break my silence once more on the subject,
Dear skk fic writers who sustain me and who I love so dearly,
The top of Chuuya's head arrives at about mouth level to Dazai. That means Chuuya's face aligns with Dazai's collarbones. On average, the arm reaches the crotch at ~wrist-level (get up and check or pull up a picture), so here we can see Dazai's crotch reaches the top of Chuuya's hipbone, while Chuuya's reaches the top of Dazai's thigh.
And because I'm thorough I brought other examples, just in case this was somehow a fluke or their height vary:
There are slight variations (also perspective and such) but it's pretty consistant. Their height difference is 21cm, it's slightly less than a head's worth, so it all tracks.
This is a great reference for their (oh so beloved) height difference for everyone, but you know why I'm saying this. You understand why I tracked their hip/crotch height. If this concerns you, you know. You can still make it work. You just need to be slightly more creative. There are easy answers. I believe in you.