add to functions.php:
Code:
add_theme_support( 'post-thumbnails' );
add_image_size( 'homepage-thumb', 500, 2000 ); // 500 pixels wide by 2000 pixels tall, soft proportional crop mode
Inside index.php put:
Code:
<?php if ( has_post_thumbnail() ) { the_post_thumbnail( 'homepage-thumb' ); } ?>
That is in place of your normal thumb. Then it will be smaller.