Karenxxx on "Header Size"

I would like the header on my site http://carrotsandclaret.com to be half the size... please could someone tell me how to do this.. Im not very good with code.

I would definitely want to do this on the home page..if I change that will all the headers be half the size (if this is the case I would still do it) or can they vary.

I do have a childs theme but Im a bit confused by all that!

Thanks