One aspect of virt-manager that I've found to be annoying is that it preallocates the storage for the boot disk of the VM, I'd rather have the space there for the VM, but not taking up space on my host server.
Here's the command that works great for making a non preallocated disk on linux. This command will create a qcow2 disk in the directory /mnt/md1/virtual-machines/ with 25GB of available space to the guest OS. You can change preallocation to on and that will create a disk in the directory you choose that has the full space that it's already taken, or you can leave it off which will still allow the guest to use 25GB of space, but it wont occupy that much space on your host OS. If the directory requires root privileges, either run the command as root or add sudo before command.
You can also specify 1T for 1TB.
qemu-img create -f qcow2 -o preallocation=off demo.qcow2 25G