First off.

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.