Clearly during this event when he's accidentally raising zombies is the best time for Dean and Rose to meet |D

Steve will be aware something's up with him when he wakes up tiny but he'll probably assume the serum finally faded until weird(er) stuff starts happening. They'll definitely have to give each other advice, preferably before we have a tiny asthmatic grimdark Steve running around---this was supposed to be a serious reason but I'm laughing too hard.