Use Add_image_size in your functions.php
add_image_size( 'custom-size', 800, 600);
in your code above use something like:
wp_get_attachment_image( 42, 'custom-size' );
Add_image_size adds a new option when you upload an image, which will then crop the image to the dimensions you need, so you would have thumbnail, medium, full size and your custom-size.
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