RightSaidJames

The width and height of the header image is hardcoded into the theme. To change this, you could amend the following lines in the inc/custom-header.php file:

add_theme_support( 'custom-header', apply_filters( 'twentyfourteen_custom_header_args', array(
		'default-text-color'     => 'fff',
		'width'                  => 1260,
		'height'                 => 240,
		'flex-height'            => true,
		'wp-head-callback'       => 'twentyfourteen_header_style',
		'admin-head-callback'    => 'twentyfourteen_admin_header_style',
		'admin-preview-callback' => 'twentyfourteen_admin_header_image',
	) ) );

The full path to the file is /[WordPress directory]/wp-content/themes/twenty-fourteen/inc/custom-header.php.

To avoid this modification being overwritten by theme updates you should use a Child Theme, paying special attention to this section to ensure you correctly load in the child theme’s version of this file (i.e. not the parent theme’s version).

To make this work, I would assign a CSS class to the ‘White Text’ style, then use either WordPress’s Editor Style feature or TinyMCE’s content_css option to load in a custom stylesheet for styles within the editor itself.

Edit:
This page gives some examples which include applying classes to TinyMCE custom styles.