I want to change the dimensions of the custom header to 960px width, 295px height.
This is done in a child theme of course.
I see Twenty Thirteen has the function twentythirteen_custom_header_setup in custom-header.php in the inc/folder.
I have copied this to my child theme folder, and changed the 'height' and 'width' values on lines 29 and 30.
But this doesn't seem to have any effect. The crop tool still seems to be configured for the original height and width of TwentyThirteen