different questions responsive pro

Hi, I've just download the pro version. I have some issues I can use some help: - how can I make a child theme? - can I use the child theme for the multisite I have? - I use the slider: is there a standard size to use for the images, now my slider have different sizes, according the image I download for the different pages Sorry if I don't write English so good, I'm Dutch Thanks for the help!