Does size almost always determine sex?

[bobsmith2002]

I bought four white silkie chicks from a feed store that were hatched on 7/23. Three of them are roughly the same size, but the fourth one is the smallest. Does this mean the small fourth one is a female and the three larger ones are males?

 

[SilverBirds]

Not really. It could be a few days younger than the others or just a bit runty. Silkies are notoriously hard to sex until they're a lot older and the best way to tell is by comb development.