In our documentation guides we have a section that explains How To Add A New Virtual Disk?. This is the easiest way of increasing the disk space available on your virtual machine.
Did you installed your virtual machine using a OVA? Could you restore it to a consistent state or download it again?
Once this is done, first of all, remember to stop the services:
sudo /opt/bitnami/ctlscript.sh stop
Then, follow the steps detailed in the previously mentioned guide. You will need to add a new disk to your Virtual Machine, format and mount it.
For simplicity, we recommend you to move the whole
/opt/bitnami directory to the bigger disk:
sudo mv /opt/bitnami /mnt/disk/bitnami
sudo ln -s /mnt/disk/bitnami /opt/bitnami
Finally, start the services:
sudo /opt/bitnami/ctlscript.sh start
Are those steps a valid option for you?