I think The Stand is the longest book, especially the uncut version. And I’m not sure what the longest movie is, but probably It.

The Stand is his longest book

I think the longest movie is also THE STAND since the old version is divided into 4 parts an hour and a half each part (6hrs total)

The new version longer.

And if we go that way, we then have to consider Mr. Mercedes and Castle Rock, for example.