Via The Morning News, Garry Wills provides some notes on how Shakespeare (and his contemporaries) structured plays to fit the company of actors at his disposal.
The trickiest job was to write for that rare commodity, the boy actors who played women. These were hard to come by and train in the brief time before their voices broke. That is why women’s parts make up only thirteen percent of the lines in the plays. The playwright had to know what stage of development each apprentice had reached. There were usually just two or three boys in the public plays (though more were available from choristers when a play was given at court or in a great family mansion). The boys’ memories were such that Shakespeare wrote shorter parts for them than for adult actors—an average of three hundred or so lines to the adults’ 650 or so lines per play. But when he had a spectacular boy like John Rice, he was able to write as big a role for him as that of Cleopatra (693 lines).