I had to google that one a few months ago.

(In the VM tab) You have to click the VMs name. Now you see the "Disk Devices" underneath it. While the VM is stopped you can click the capacity and change it.

It's worth noting I had to hit "ENTER" to save the change in disk space. Simply clicking out of the box with my mouse wouldn't save anything. Took me longer than I want to admit to figure that out...

4 years later, thanks as well!

Thanks, I don't know why this is "hidden" in a very not obvious place.

Stop the VM, go to the VM tab in the GUI and expand the disks. Where it lists the capacity, click it and it'll become a text box which you can enter a new larger size (can't shrink this way) Then start the VM and expand the partition into the new free space.

If that doesn't work then from command line

qemu-img resize [vdisk path] +10G

to add 10GB to the size, or specify the new total size without the plus, for example:

qemu-img resize vdisk1.img 120G

will set vdisk1.img to 120GB

That doesn't work in this application because I have manually assigned the vdisk rather than letting it default to "auto".

In the "Edit VM" option the size is unavailable, but u/the_harakiwi answered it:

(In the VM tab) You have to click the VMs name. Now you see the "Disk Devices" underneath it. While the VM is stopped you can click the capacity and change it.