To determine the correct size, you need a measuring cup and a (kitchen) scale. Place the measuring cup on the scale and fill it until you reach the weight you want to pack. Now, check how many milliliters the measuring cup is filled with, and you'll know which size you need! See the steps below for visual examples.
 

Step 1

Place an empty measuring cup on a scale.
 

Step 2

Fill the measuring cup with the total weight of the product you want to package.
 

Step 3

Read how many ml there are in the measuring cup. Now you know which size you need!