Multiple Post Thumbnails force size

I am working on a site that uses multiple post thumbnails. The thumbnail size for single posts is 306×306. Some of the thumbnails are too small and I want to force the size. Here is the code that pulls in the thumbnails:

<div class="content-image">
                        	<?php if ( class_exists('MultiPostThumbnails')){ ?>

                            <div class="img">
                              <?php
                                    for( $i=1; $i<=5 ; $i++ )
                                    {
                                        $image_id = 'featured-image-'.$i;
                                        $image_url = MultiPostThumbnails::get_post_thumbnail_url(get_post_type(),$image_id );
										echo $thumb_id = MultiPostThumbnails::get_post_thumbnail_id(get_post_type(),$image_id,$post->ID);

                                        if( $image_url != '' )
                                        {
                             ?>
                                            <a class="lightbox" href="<?php echo $image_url;?>" style="position: absolute; top: 0px; left: 0px; display: none; z-index: 5; opacity: 0; width: 306px; height: 306px;">
                                                <span><img width="306" height="306" alt="" src="<?php echo $image_url;?>"></span>
                                                <div class="sprite"></div>
                                            </a>
                            <?php
                                        }
                                        else
                                        {
                                          break;
                                        }
                                    }
                            ?>

                            </div>

There are two places where image size is mentioned but I can’t figure out how to force the scaling.