To find the right size, you will need a measuring cup and a (kitchen) scale. Put the measuring jug on the scale and fill it until the scale reads the number of grams you want to pack. Now see how many ml the measuring cup is filled to and you know what 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 pack.
 

Step 3

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