valanc'd: i.e., fringed with a beard.
Both here and in the next two sentences, Hamlet is talking to young men who he has seen play women's parts when they were even younger. But now one of them has grown a beard, so that he might challenge another man to a fight, giving Hamlet an opportunity to joke, "comest thou to beard me in Denmark?" And the other is quite a bit taller, at least as tall as the women he used to play.