How do I set a specific size for my image?

Use Add_image_size in your functions.php

function theme_sizes(){
 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