Things to consider when designing for flexible materials (+ dimensions of a 2,5 Kg chicken bag?)

At work I have been asked to make a design for a 2.5kg Chicken Bag. I have seen some examples on google but how do I know how many inches should the bag be?

And my another question is: If I am designing for a plastic bag, what are some things I need to keep in count?

Here is the example: