I think Faramir may be on to something.
Baensch has them at up to 5, The Barbs Aquarium has them at up to 5.5", usually smaller (why I figured 4 as an average). Fishbase, which I always associate with wild specimens, has them at 5cm (2"). So then I opened up Gina Sandford's Aquarium Owner's Guide: "Size 2 in (5cm) although wild specimens can grow as large as 6 in (15 cm)". Mystery solved, and a new fish added to my candidate list.
And Kelbel, you may giggle now but you'll soon find yourself driving past fishstores (that aren't on the way home).
How does that song go? "A 10 is really much too small, I need a 29 or a 30… no, a 55. And if I'm going to get a 55, then why not a 75?"