Magic fields doesn’t necessarily attach images to posts when uploaded via the WP image uploader, and it’s not so simple to get the ID associated with an image. The plugin has its own function to retrieve different sizes:
$image_attributes = "w=250&h=150&zc=c&q=90"; // 250x150px, crop to center, quality 90
echo get_image('my_field_name',1,1,1,NULL,$image_attributes);
New sizes are generated on-the-fly and then cached for future use. See get_image() in Magic Fields wiki.