REGISTER TO REMOVE ADS
Or Roxas, since he is still kid too XD
Roxas=Sora's height=pretty close to Kairi's height (exact, if not taller). Since Xion is Kairi's model, and Roxas is almost Sora's, Xion is to Roxas as Kairi is to Sora (height-wise. Hopefully).
Zexion was taller than CoM Riku, wasn't he? For some reason I've always thought of KH2Sora being about KH1Riku's height. But I never got to see Zexion around Sora or Roxas, so, can't say for sure.