I just updated the code that you provided, this code you have is not printing out the description, it is just limiting the size of it. You will need to set this value to a variable and print this out to see it, you can set it to the string var and just add it to the echo on the next line like this:
<?php
$string = "" . $row_WADAProducts['ProductShortDesc'] . "";
$max = 100;
$string = substr($string,0,strpos($string," ",$max));
if(strlen($string) > 100){ echo $string . ". . .";}
?>