To estimate the shrinking you usually do a test swatch that you felt.
Like make a 4' X 4' swatch and then measure again after the felting procedure.
If your like me and like to be surprised(or don't have enough yarn to do a test swatch) just make the bag about 50% bigger than the smallest bag you would like.
I don't think that there's a yarn that shrinks more than 50%(others might prove me wrong here
) and so you will at least end up with an appropriate sized bag.
As for the Landscape: I found a similare yarn over here and knitted+felted a bag with it. You can see the result here
. It shrank about 40%.
The shrinking percentage is different from one yarn to another and it also depends on how you tread it...