What gets me more than how long they get is how thick they can get. I was in a LFS that had a CAE in their planted tank that was as thick as my thumb and every bit of 6". It looked like a giant underwater slug. Disgusting. But, I will admit I am more than a little prejudiced againt CAEs because I was sold 4 under false pretenses (thought I was getting otos) and then one had the audacity to murder the other 3 plus 3 frogs and a gourami (I firmly believe since their mysterious deaths coincided with the CAEs arrival and after its consequent removal to a small breeding trap, my personal version of an underwater KY State Penitentiary, the deaths stopped.) Okay, end rant.
In all fairness it did a good job of clearing up an algae problem I was having, but soon it turned its sights toward its tankmates. I used a variation of a trick to catch it that Somonas I think posted in this forum. I put an algae wafer in my net and held it close to the bottom. When my CAE was in there far enough I got him out.