@Geohie You saying she is 6'3''? I checked too and her height isn't consistent throughout the drawings. When she stood at the door, assuming it was close to 7ft, I guess this isn't too far off. HOWEVER, other parts when she is sitting or other scenes she appears to be 6'. Nature at the biological level says she as a woman is done "growing" in height at 15, so we should assume the author intended for her to be normal size but to look like she towers over everyone else.
The boy, is far too small, as a few scenes he is made out to be under 4ft, which would be literal "midget" tier as to be classified as a "dwarf" his body would have to be disproportional which it clearly is not. While there are small/short men, at the age of 14 there is a chance he has not hit puberty yet due to lack of nutrition. However, if this story continues his shortness past 16, then the puberty angle wouldn't really be logical that may turn into a plot hole.
TO FURTHER ADD, all muscle is typically proportional to height. This doesn't seem to be the case IRL because people view defined muscle as big muscle, despite the obvious difference in mass. It's why a lanky dude has more lifting/pushing power than a short "built" dude.
This trope can fall into a category of "small man" syndrome, that supposes that a man becomes hyper aggressive due to disrespect from other people.
Japanese manga being Japanese manga makes it weird, as we finally get a "normalish" non-weak self confident MC, but sadly he is a short boy.
The author though, maybe didn't think of any of this, they just wanted to write a "good male lead" but in an attempt to make it "original" and target a certain audience the trope is simply "alpha shota".
I may be over thinking this, but the story will probably just tease the readers for a while until we get bored and end up getting closure 5 years from now on some irrational ending.