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