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Can I only activate components I need?

Sure! By default, all components are enabled. If you have [KC Settings plugin] (http://wordpress.org/extend/plugins/kc-settings/) installed and activated, you'll have the luxury to select the components you need by visiting Settings » KC Media Enhc. in you dashboard.

If you don't want to use KC Setting plugin but still want to enable only certain components, you'll need to add this block of code to your theme's functions.php file and change each unwanted component's value to false:

function my_kcme_options( $options ) {
    $options = array(
        'general' => array(
            'components' => array( 'insert_custom_size', 'taxonomies' ),
            'taxonomies' => array( 'category', 'post_tag' )
        )
    );

    return $options;
}
add_filter( 'kcme_options', 'my_kcme_options' );

Please replace the taxonomies array with the taxonomy names you want to set for the attachment post type.

Options saved by KC Settings will always get the highest priority when the plugin is active.

I Don't see the custom size option when I tried to insert an image into posts

Either the original image dimension is smaller than the custom size, or you added the custom size after the image has been uploaded. If this is the case, you need to rebuild the image's thumbnails using Viper007Bond's excellent plugin: [Regenerate Thumbnails] (http://wordpress.org/extend/plugins/regenerate-thumbnails/)