add image sizes

I likie to add some other image sizes becaus the slider need 1200×5000 and the featured image needs 270×250.

How must I do that in the functions.php?
e. g.

function <strong>whatmustbehere</strong>_setup() {
	/*
	* Registers a new image size
	* 1. parameter  - name of the image size
	* 2. parameter - picture width
	* 3. parameter - picture height
	* 4. parameter - crop (true or false)
	*
	* uncomment the line if you want to use it
	*/

	add_image_size('featured', 270, 250, true);
	add_image_size('slider', 1200, 500, true);
	add_image_size('small', 60, 60, true);
	add_image_size('postimage',220, 180, true);