Control resizing of uploaded images

Why not set your own image sizes from within your theme in functions.php?

add_image_size( 'some-image-size-name', 500, 300 );

Then you can call that size you want from your template.

<?php echo get_the_post_thumbnail( $post_id, $size, $attr ); ?> 

Links

Add image size: http://codex.wordpress.org/Function_Reference/add_image_size

get post thumbnail: http://codex.wordpress.org/Function_Reference/get_the_post_thumbnail