They are very efficient at any size. How big it will grow depends on what kind it is, i'm not very good at telling them apart, but if you could come up with a pic i'm sure someone could help you better.
I've only had mine for a couple of months, so I can't really comment on the growth rate. I can tell you that they are very efficient algae eaters. I have a 120g Malawi tank that has about 300 lbs of rock. It had developed a significant amount of brown algae on the top surfaces of all the rocks when I introduced the BN (female about 2.5"). The algae was completely gone 48 hours later and there's been no sign of it since.