Mine was 2 inches and it was chasing and harassing my 12inch oscar. He was a mean fish, then one morning, my oscar was dead on my floor. MUst of jumped to get away.
i think it varies...
i had GTs that were around 4" TL and they held their own in my mish mash tank last time of convicts, oscar, some other odds and ends and tried to breed!
Mine started getting pritty agressive at about 6 inches. He easily dominates the two Oscars in my tank that are almost twice his size. He hasn't done very much harm yet though, just removes a scale or two every now and then.
I have to breeding pair and both my pairs have been aggressive both male and female my current pair @5" are reasonable they live with a sun catfish, oxidoris niger, and a pleco. but my first pair I got tired of them killing everything finally I dropped them in my 240 with my wallago attu who wound up showing them who was the alpha fish.
it must just depend on the individual fish. I have 2 GTs which are about 3.5" and they are wimps...all the other cichlids i have show em who's boss. But my cuz has a GT which is about 10" and is in a 190g. which it pretty much owns the tank. so it all depends i think.
i think my small green terror likes my 3in. jag. my jag hides all day in his flowerpot and nobody gets in, except for my gt. its like they enjoy hiding together. they hate everyone else in that tank, its tight.
My green terror is a wuss. He holds his own but is docile as they come and fairly skittish. He gets behind my jag sometimes and flares at him, when the jag turns around he swims away. My GT even lets my con who is smaller show him who the boss is sometimes.
I think all Cichlids are different,its like rolling the dice when you combine them. Mine would get along fine for years and one day just tear each other up for some unknown reason. All you can do is try and watch them for injurys.