How to get specific image size

Hi, i included a image with link with the following code

<a>"><img class="slidimg" src="<?php get_image_url(); ?>" alt="<?php the_title(); ?>" title="<?php the_title(); ?>"/></a>

This is working fine, but how can i tell him to use a specific image size that i had definied in the functions.php? With this code above i only get the fullsize image.

Thanks a lot