almost free,
I work in an interior decorating showroom here in Montreal and one of my task it to oversee the production of decorator pillows for showroom sales.... (and I use this rule for all my own pillow making as well)....
We order all of our pillow fillers (feather), to be bigger than the pillow covers...
Example: If I have a pillow cover that is 20" x 20", I order fillers that are 22" x 22"....
An exception would be a very lightweight silk - I would order 21" x 21" instead.
For synthetic fillers, it may be a bit different - but it will depend on how densely the pillow forms are filled.
Enjoy the pillow making....
Bill