Thank you so much!
Dean might try to run off at first, much like Sam, but once he’s over the initial startling first impression of everything around him becoming giant, he will do his best to hide his nerves for Sam’s sake. His little brother will be fret for Dean, considering he was out cold for a week, and now he just tried to run from Sam.
As for how they’d handle it afterwards, Dean is still the older brother, and still in charge of Sam, but Sam’s going to have to learn how to defend them both. Dean will help him learn how to care for his own gun, going so far as to help Sam clean the gun. Sam’s not sure how much Dean helps, since he has to make sure to steer clear of his older brother’s tiny body, but it certainly seems to cheer him up.
Honestly, they’ll bicker just as much since Dean’s a bossy little smol, but Sam will hate if he ever accidentally startles his older brother, or god forbid, scares him.