It depends you can't just go by size alone, age also comes into play. I have heard different sizes and ages so I believe it will vary. Generally I have heard 6 inches and roughly 4 years old is when they may start to breed as it is when they should be reaching sexually maturity - however those are also not the only factors you can depend on and they can also vary. Water quality, tank mates, tank decor along with other conditions can have an influence as well (maybe not in all cases, but from what I have heard). Those that get frontosas to breed usually have a tip up there sleeve they conveniently leave out when talking with noobs or they would rather you figure it out on your own and do research (so always take what you hear with a grain of salt. Do a lot of research on it.). Others I have seen don't really put much effort in and have had frontosas breed. Usually the first time they do you wont get a lot of fry either. When they are ready they will do their thing, if they don't then do some research try changing something around. I have a colony of 12 ranging from 4 - a little over 6 inches right now. I have had a one of my larger female fronts show signs, but nothing has come of it yet (I had a picture but I can't find it =[ ) I will have to try changing things to see if it helps or not. There are lots of factors to take into account what works for one - won't always work for others.