Maybe just to add: I started the droplet from scratch and it should have been the complete size... I'm taking a snapshop now and seeing if I can rebuild it...

Nope, after a new droplet using the image (with virtio eabled):

Filesystem 1G-blocks Used Available Use% Mounted on
/dev/vda 20G 19G 1G 100% /
udev 16G 1G 16G 1% /dev
tmpfs 7G 1G 7G 1% /run
none 1G 0G 1G 0% /run/lock
none 16G 0G 16G 0% /run/shm

If I look in the control panel I do see: Size: 32 Gb | Disk: 320GB
But I do not understand how I can make the size available to my droplet? :(

This is an issue that we ran into where if the original create failed for a larger sized droplet it would try to create the same RAM/CPU with a smaller disk drive as a failover but I think this isn't clearly communicated so we will be looking to switch away from this and if the create for a larger sized volume fails it will just communicate that it was a failure so you can retry.

This was an error on our part by not providing enough transparency in the create process.