There was a guy from GVZ who already went ahead and put up a download of his modified custompresent.pak file. For us, that means we don't have to go through the same tedious process as he did and we can just tweak values.
I've put up my modified custompresent.pak on you-know-whatspace for the time being so you can grab it there. Can't enter the link here but when on the default url of it add the following for the file: /file/742x63
Overwrite the custompresent.pak found in the NCSOFT\Aion\data\custompreset folder with the one from the dl. Once done, open the archive with winRar or similar, find and open the bodytypes.xml file with WordPad. Personally, I just tinker around with values before saving it (overwrite/update archive). It's the display and scale bone values that you will want to focus on.
As said before, however, Aion models tend to have some poor geometry. There's only so many increments you can do before it looks like crap.
You'll also need to launch Aion from a .bat if using a modified custompreset.pak or else the game will auto-repair the file (a good solution for if you want the default values again).
Hope this helps.