General minimum size for any fish that can express the fader gene is 1.5". However, when dealing with fish that fade(RT, ZZFaders etc), it can take them years to fade, some never fade.
I think a better question may be what is the size when you know they won't fade. Red texas with weak fade gene can fade at 10 in +. It can take some longer.
Usually the ones with less pearl can fade younger, my fry the total faders are fading at 2 in.
But any full red fully pearled red texas under 3 inches may have been given hormone. It is important not to trust people who have full RT's that are too small. If you want a healthy high quality red texas, buy a full fader around 5-6 inches.