Adjust header height on web-based WordPress

Hello,
I have found a number of topics pertaining to header height, most of which involve solutions pertaining to lines of code in a css file. I am using WordPress provided by Machighway hosting for my new blog. As such, I don’t have WordPress on my hard drive and don’t have all those files to which most solutions refer, or if I do, I have no idea where they are. I installed the plug-in to create a child theme of Twenty Eleven (Son of 2011). I would like to narrow all that white space above the image in the header. Can I do that? If so, please explain it like you’re talking to a 7 year old in hopes that I’ll understand.
Thanks in advance for your time.