For any media mail package, I usually don't explicitly measure dimensions and just randomly specify some smaller numbers. Most of the packages I use are pretty universal. But one nice thing about media mail is it's not very hard to arrange shipment as it's just by weight, as long as it's qualifying material. Most of the time, DVDs or CDs aren't even an issue, weight-wise. Books sometimes will vary widely enough, but none of it is ever too hard.
