I found it by myself.
This is how I did (and it worked for me):
Go in to the template file “function.php” and search for “define( ‘HEADER_IMAGE_HEIGHT'” and then change the value 105 to the size you want.
(In my case 150 or 200px)
But remember to keep a record of the changes you make in the template file. Because if the theme gets upgraded your changes will be lost and You might not remeber where you did all your changes. ??
