The compression stuff sack provided with your One Planet sleeping bag has been designed to protect your bag from over compression. On the right is the Robin Hoodless +3 stuff sack.
However, when storing your bag for extended periods, it is important to allow the bag to expand to its full size. Current One Planet down sleeping bags come with a cotton storage sack which allows the down to loft to its full potential, preventing potential damage. You can also hang the bag in a cupboard or keep it on a large shelf: the most important thing is not to keep it in its stuff sack.
Note: synthetic bags do not come with storage sacks.
Stuff sack dimensions (not compressed, approximate)