((Well, look who got some inspiration and actually wrote a fic! This is my entry for the @brothersapart tumblr contest and is based on this post. I’ll probably upload it to Ao3 sometime later. In the meantime, enjoy.))
Well, there’s not much he can do. If it’s at Bobby’s, he’ll probably sulk while she shrinks down. If she’s not at Bobby’s, he might be able to get the hunter to tow the car there.
If Bobby’s busy, Dean will sit there, watch her shrink, and give his angry face to anyone that gets too close. That’s his crazy car.
Also, there’s a huge difference between watching someone drive or learning what the controls are, and actually driving, which is why a lot of time is spent with either driving teachers/your parents in the passenger seat to talk you through those first attempts.
A. He’d have a heart attack.
B. He’d have a panic attack.
C. OMG Sam what the hell is happening. What. Is. Happening.
He’d only be able to be in the car briefly if it was a slow shrink, cars really aren’t made for people relatively over 7, 8, 9… 20… feet. Sam wouldn’t mind learning to drive, though it’s hard for Dean to show him what to do, and Dean would fret the entire time. Though Sam already has a decent grasp of the perspective after seeing how Dean looks at the same scale.
That would be a very painful time for the two brothers. Not only is Sam shrinking and they can’t stop it, there’s no way of knowing if it’s going to stop, like in my fic The Incredible Shrinking Winchester.