Honestly, it depends on the fish.. I've had some males that looked really good as early as 3 inches, but usually they don't start to look really good until they get to about 6 inches and up. I do agree with the other posts that the bigger they get the better they tend to look. But when they get really huge and old, to the point where they start to swim slower I don't think that they look as good anymore, but that's just my personal opinion.. Here's a vid of one of my current males that's about 6 inches now:
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And here's a vid of my 13 inch male:
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