I'm contemplating buying this for a school backpack in a rainy climate. The knapsack in question seems to be roughly 20x14x12...will a Large cover be unhelpfully loose on a stuffed knapsack, or will the Small/Medium somehow cover enough to keep all the papers/laptop dry?

Tech Specs:

Material:

190T nylon taffeta 

Weight:

(xs) 3.84 oz, (s) 4 oz, (m) 4.16 oz, (l) 4.32 oz 

Recommended Use:

backpacking 

Manufacturer Warranty:

lifetime