Changing the huge header image size in 2015 twenty eleven template

I have read countless threads on how to do this but it appears to me that all of the threads I have read are old and there must have been a revision to the functions.php file for the twenty eleven template.

My goal is to first off figure out how to resize the huge 1000 x 288 header image.

Can anyone tell me what exactly to copy into my blank functions.php file located in the child theme to make this happen?

My success so far is limited to having wordpress customize header image option at least recognize my 648 x 153 image size, but the image is still stretched out to 100 x 288. When I say it recognizes the size that means that the wording in the customize section says:

While you can crop images to your liking after clicking Add new image, your theme recommends a header size of 648 × 153 pixels. So I’ve done something, but there is something still overriding the size and forcing it out to 1000 x 288.

I’ve watched you tube videos and read multiple threads but when people show text copied from the functions.php, that text simply isn’t located in my functions.php. They must have updated.

Any ideas on exactly what to copy into my blank functions.php?