Is it possible to change the size of a Virtual Machine's disk?

Yes, it is possible.

Volume-backed Virtual Machines

For Volume-backed Virtual Machines, you need to expand the volume:

1. Click the connection icon of the Virtual Machine;
2. Next to the desired volume, click the Expand button;
3. Enter the new volume size and click Expand;
4. Wait until the status changes to In Use;
5. Reboot the Virtual Machine using the Reboot command in the Actions dropdown menu.

Local Virtual Machines

For Local Virtual Machines, use the Resize option in the Actions dropdown menu and select a configuration with a larger disk size.