change header size in twenty eleven

Resize the Header in a Twenty Eleven Child Theme

need a little more than this…
this means I need to make a functions.php?
what do i write in it?
how do I get it to get the info from twenty eleven?

this means I need to make a functions.php?

In your child theme’s folder, yes.

how do I get it to get the info from twenty eleven?

All you need to add to your child theme is explained in that article.