In my opinion, it is realistic to have 15ft. However they would have to accommodate every area for it, with size in mind. VO would just be a place with very high ceilings lol. The issue I have heard in FFXIV regarding the small avatars was, bigger ones would have been a pain during combat. That was not the case in FFXI; titan was pretty big, people just fought around him. In CO, folks can also get very big for a short time in combat, just fight around them. No biggie.
Ark is pretty much big dinos clogging up the screen during battle although you are mostly at a distance.