I don’t usually estimate Cd alone; I usually just look at CdA. For a “normal-sized” rider on a modern TT bike in an ideal position, CdA around 0.20 m^2 is pretty attainable. For a small-sized male rider, I’ve seen around ~0.18. I’ve seen some data for a small-sized female rider on a UCI-legal pursuit bike below that, but I don’t know what kind of power meter was being used so I don’t know if the estimate is reliable.
As a rough rule (just to get you into the right ballpark) CdA appears to scale with mass less than linearly. Andy Coggan used to use a rule of thumb of around mass^0.7 for fixed height – but that rule of thumb has a lot of noise in it. Nonetheless it appears that the exponent < 1.0.