I think there might be a couple of different species, one grows to about 8-9" and the other to 14"+.
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I think I might have the smaller species, because he hasn't put on much size at all for the past 6 to 8 months. I've seen clips on youtube of some behemoth flagtails, and I'd actually prefer mine to be the smaller variety (I know, not very MFK to say that).
The tank kinda distorts the actual size of the fish, but he's every bit of 8", possibly 9".
I lost an Arulius Barb a few months back and was shocked when I measured him at almost 5"... eyeballing him, he looked to be about half the size of the Flagtail.