DNA is really the only reliable way.
And there are two sub-species of Congo African Grays (i.e. red tails)
The smaller birds come from the coastal regions of Africa. (aka "coastal" or "normal" grays) (350-ish grams)
The significantly larger birds come from the interior regions of Africa. ("Silver" grays) (450+ grams - My "silver" gray is PRETTY HUGE at 560 grams)
So, size isn't the indicator of the bird's sex, it's the indicator of where the bird's parents were captured and imported from.
Do a DNA test to find out if it's a boy or a girl.