Rumiko Takahashi has to draw/plan the entire comic by herself in one week's
time, so mistakes can be made. I looked over the first few comics since I
figured that this is where Rumiko Takahashi would make the least mistakes not
being accustomed to drawing the characters yet. Akane is an inch or so taller
than Nabiki, while being a bit shorter than Kasumi. Akane only comes up to
Ranma's nose when he's a guy (remember this is anime so the hair is quite
large so there's a few inches there). Ranma-chan seems to be about the same
height as Nabiki, maybe a bit taller. So why is Ranma-chan seem taller than
Akane? I'm going to venture that this is because Ranma-chan stands on her toes
a lot, possibly to make up for her lost height. When Ranma-chan is fighting
(and has her feet planted) she is shown as shorter than Akane (I'm looking at
the first Ranma-Ryoga fight here, the one where Akane's hair is cut). While
when Ranma-chan and Akane aren't in combat Ranma-chan is drawn taller or the
same.