Source

File: wp-admin/includes/class-custom-image-header.php

final public function get_header_dimensions( $dimensions ) {
		$max_width       = 0;
		$width           = absint( $dimensions['width'] );
		$height          = absint( $dimensions['height'] );
		$theme_height    = get_theme_support( 'custom-header', 'height' );
		$theme_width     = get_theme_support( 'custom-header', 'width' );
		$has_flex_width  = current_theme_supports( 'custom-header', 'flex-width' );
		$has_flex_height = current_theme_supports( 'custom-header', 'flex-height' );
		$has_max_width   = current_theme_supports( 'custom-header', 'max-width' );
		$dst             = array(
			'dst_height' => null,
			'dst_width'  => null,
		);

		// For flex, limit size of image displayed to 1500px unless theme says otherwise.
		if ( $has_flex_width ) {
			$max_width = 1500;
		}

		if ( $has_max_width ) {
			$max_width = max( $max_width, get_theme_support( 'custom-header', 'max-width' ) );
		}
		$max_width = max( $max_width, $theme_width );

		if ( $has_flex_height && ( ! $has_flex_width || $width > $max_width ) ) {
			$dst['dst_height'] = absint( $height * ( $max_width / $width ) );
		} elseif ( $has_flex_height && $has_flex_width ) {
			$dst['dst_height'] = $height;
		} else {
			$dst['dst_height'] = $theme_height;
		}

		if ( $has_flex_width && ( ! $has_flex_height || $width > $max_width ) ) {
			$dst['dst_width'] = absint( $width * ( $max_width / $width ) );
		} elseif ( $has_flex_width && $has_flex_height ) {
			$dst['dst_width'] = $width;
		} else {
			$dst['dst_width'] = $theme_width;
		}

		return $dst;
	}