This is for everyone who’s been trying to find out how to get the medium image size of a post attachment in WordPress:
You’ve got to send the function the ID you’re trying to get the medium images for.
$medium = wp_get_attachment_image_src($picture->ID, 'medium', false);
$med_url = $medium;
$med_width = $medium;
$med_height = $medium;
There are a lot more things you can find out by analyzing