Volumes can be managed on "Volumes" tab. As shown in Img. 1, you can review details of existing volumes and/or add new volumes, which can be attached to virtual servers.
When you create new volume, you can choose one of available types from list of Volume Sources (see Img. 2)
You can create blank volume and then attach it to existing server, or build new volume from image or previously created snapshot.
Next step is to provide desired size of new volume in Gigabytes. (Img. 3)
After submitting the form, you'll see message that volume is created and you can review its details as shown in Img. 4