All user accounts come with 2 GB of storage free of charge. In IMAP parlance, this is called “quota”.
If you run out of storage, or need extra storage, you can purchase more space in the Cockpit in “Account” -> “Edit Account Settings”. The price will be updated in real time accordingly and once saved, will be reflected on your next invoice.
At the time of this writing, you may increase (or decrease!) in 1 GB intervals. Each additional 1 GB will charge you an approximate CHF 0,50 per month.