I’d originally allocated the default 8G to my primary partition when installing NixOS. I ran out of disk space when I tried upgrading to KDE 5. This is how I increased the size of the partition.

1. Shutdown your VM.

2. Use VirtualBox tools to increase the size of the VDI.

$ VBoxManage modifyhd NixOS.vdi --resize 20000

3. Boot the VM from the NixOS Live CD.

4. Login as root.

5. Resize the partition using fdisk.

You have to delete and recreate the partition. In my case, I’ve only got 1 partition, so it’s partition 1. Here I enter the commands: ‘p’ (print the partition table, ‘d’ (delete the partition), ‘n’ (create a new partition), 4 × ‘<return>’ (accept default), ‘w’ (write/sync the partition table to disk).

$ fdisk /dev/sda
Command (m for help): p
/dev/sda1     2048 24575999 245739952 11.7G 83 Linux
Command (m for help): d
Selected partition 1
Partition 1 has been deleted
Command (m for help): n
Select (default p): <return>
Partition number (1-4, default 1): <return>
First sector (2048-40959999, default 2048): <return>
Last sector, +sectors or +size{K,M,G,T,P} (2048-40959999, default 40959999): <return> 
Created a new partition 1 of type 'Linux' and of size 19.5 GiB.
Command (m for help): w

6. Resize the filesystem.

$ mkdir /mnt/root
$ mount /dev/sda1 /mnt/root
$ btrfs filesystem resize max /mnt/root

7. Reboot and boot from the drive.

$ reboot