I don't know if it's the same for all fish, it probably just relates to mine.
I think he's in the 8-9" range now. I can't get a good side profile measurement since when you're near the tank he goes head on towards you.
He was very frustrating for the first 6-7 months I owned him. Never eating and hiding all the time. Now he's a pig thankfully.
He's not quite alone, he has a dozen tiger barbs and a few left over baby HRP's from a spawn in our other tank. (If anyone wants them let me know, there are three left).