Note: Instructions are based on the default “TwentyTen” Theme
(default size is 940×198)

1- Open your file “functions.php” in a text editor
(will be located in your theme)

2- Find the following code:

define( ‘HEADER_IMAGE_HEIGHT’, apply_filters( ‘twentyten_header_image_height’, 198 ) );

3- Change “198” to your desired size

4– SAVE AND OVERWRITE YOUR PREVIOUS functions.php FILE

5- Re-upload your 198 pixel height banner / header

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