I have two L191 that came in at the same size, around 2.5". 1 is about 3", other is almost 4". (you can see the difference in my pleco post). I feel that royals grows at whatever pace they want. Sometimes, they don't seem to grow at all for a year or so. Sometimes they go a period of fast growth. Agree with David R, it's much easier to feed a larger group of plecos, it's about having more food around, and more competition around that triggers them into fighting for food.
The two L191 were the first two plecos in their tank. I was worried that they didn't eat at all, carrots, wafers, pellets, anything! Eventually, I gave up and didn't bother checking on them. As I introduced more plecos into that tank, eventually they start coming out and eatting in the open. Sometimes, they eat so much that they turn into a golf ball, like the second picture in David R's post. My experience is that panaques, and pseuda's don't do well alone. Hypostomus like commons fair better as the sole bottom dweller in the tanks.
