Targeting different image size in WordPress

So I know WordPress uploads different image sizes every time you upload an image.

So for example I upload an image called background_main.jpg and in my uploads folder ‘ll have :

background_main-1024x512.jpg
background_main-150x150.jpg
background_main-300x150.jpg
background_main-768x384.jpg

So currently to pull out an image I handle this like:

$classImg  = wp_get_attachment_url( get_post_thumbnail_id($key->ID) );

Then I can spit it out by just using $classImg.

How can I pull out the same image but a smaller size for instance the 150×150 image or the 300×300 image?

Thanks!