gistfile1.php

<?php
function dm_library_document_set_file_size($pieces, $is_new_item) {
	$size_val = $pieces['fields']['size']['value'];
	if (!$size_val) {
		$attachment = $pieces['fields']['attachment']['value'];
		$attachment_id = reset($attachment);
		if ($attachment_id) {
			$attachment_id = $attachment_id['id'];
			$file_path = get_attached_file($attachment_id);
			$file_size = filesize($file_path) / 1024; // get size in kilobytes
			$file_size = round($file_size, 2);
			$pieces['fields']['size']['value'] = (string)$file_size;
		}
	}
	return $pieces;
}
add_filter('pods_api_pre_save_pod_item_library_documents', 'dm_library_document_set_file_size', 10, 2);
?>