How to change size of the place over the header image?

Do not edit the Twenty Eleven theme. First create a child theme for your changes.

I do not edit. I have just said I have modded theme. It’s under other name and my changes won’t be updated.

In that way have you “modded” the theme?

Yes. I think I am able to rename and customize it as independent.

[ Please do not bump, that’s not permitted here. ]