Resize virtual hard drive

$ VBoxManage modifyhd <path to .vdi> --resize 40000

The value provided to --resize is in MB.

Limit hard drive bandwidth

To limit a virtual hard drive bandwith (e.g. to simulate a classical SATA HDD) :

$ VBoxManage bandwidthctl "VM name" add Limit --type disk --limit 20M
$ VBoxManage storageattach "VM name" --storagectl "SATA" --port 0 --device 0 --type hdd
                                   --medium disk1.vdi --bandwidthgroup Limit