virt-install appears to convert existing  disk image files sizes to
gigabytes at around line 650 of VirtualDisk.py which has this code:
	newsize = float(newsize) / 1024.0 / 1024.0 / 1024.0
Even with floppy disk images. Whether the above assumption is actually what happens, the
consequence is that I get this error:
[Sat, 11 Jun 2011 22:02:46 virt-install 32253] DEBUG (virt-install:330) parse_disk: returning {'format': None, 'bus': None, 'readOnly': True, 'volInstall': None, 'path': '/var/lib/libvirt/images/fd.img', 'device': 'floppy', 'volName': None, 'conn': <libvirt.virConnect instance at 0x2402998>, 'size': None, 'driverCache': None, 'shareable': False, 'sparse': True}
[Sat, 11 Jun 2011 22:02:47 virt-install 32253] DEBUG (VirtualDisk:860) Path '/var/lib/libvirt/images' is target for pool 'default'. Creating volume 'fd.img'.
[Sat, 11 Jun 2011 22:02:47 virt-install 32253] ERROR (cli:196) Error with storage parameters: Size must be specified for non existent volume path '/var/lib/libvirt/images/fd.img'
[Sat, 11 Jun 2011 22:02:47 virt-install 32253] DEBUG (_util:221) Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/virtinst/cli.py", line 480, in disk_prompt
    dev = VirtualDisk(**arg_dict)
  File "/usr/lib/pymodules/python2.6/virtinst/VirtualDisk.py", line 437, in __init__
    self.__validate_params()
  File "/usr/lib/pymodules/python2.6/virtinst/VirtualDisk.py", line 946, in __validate_params
    self.__check_if_path_managed()
  File "/usr/lib/pymodules/python2.6/virtinst/VirtualDisk.py", line 864, in __check_if_path_managed
    "existent volume path '%s'" % self.path))
ValueError: Size must be specified for non existent volume path '/var/lib/libvirt/images/fd.img'

This is the actual file:
-rw-r--r-- 1 root root 1474560 Jun 11 21:46 /var/lib/libvirt/images/fd.img

Note that this floppy image was okay in the RHEL6 beta where I created it ans used it for several Debian Squeeze installs using python-virtinst-0.500.3-7.el6.noarch or (slightly) earlier.

Note, if the disk image is missing, this message arises:
ERROR    A size must be specified for non-existent disks.

It does not identify the problem file, and the text is slightly different although the meaning,
it seems, is the same.

Can you provide the full output of running this command with --debug?
Also, what's the output of

virsh --connect qemu:///system vol-list default

Can you provide the full output of running this command with --debug?
Also, what's the output of

virsh --connect qemu:///system vol-list default

It's not hard to reproduce, this does it:
  dd if=/dev/zero of=/var/lib/libvirt/images/fd1440.img count=2880
  /usr/bin/virt-install -n dummy  -r 192 --arch=i686 --vcpus=1 --os-type=linux --os-variant=debiansqueeze  --disk path=/var/lib/libvirt/images/fd1440.img,device=floppy --noreboot --pxe

~#  /usr/bin/virt-install -n dummy  -r 192 --arch=i686 --vcpus=1 --os-type=linux --os-variant=debiansqueeze  --disk path=/var/lib/libvirt/images/fd1440.img,device=floppy --noreboot --pxe --debug 2>&1 | tee /tmp/virt
Sun, 07 Aug 2011 10:42:23 DEBUG    Launched with command line:
/usr/bin/virt-install -n dummy -r 192 --arch=i686 --vcpus=1 --os-type=linux --os-variant=debiansqueeze --disk path=/var/lib/libvirt/images/fd1440.img,device=floppy --noreboot --pxe --debug
Sun, 07 Aug 2011 10:42:23 DEBUG    Requesting libvirt URI default
Sun, 07 Aug 2011 10:42:23 DEBUG    Received libvirt URI qemu:///system
Sun, 07 Aug 2011 10:42:23 DEBUG    Requesting virt method 'default', hv type 'default'.
Sun, 07 Aug 2011 10:42:23 DEBUG    Received virt method 'hvm'
Sun, 07 Aug 2011 10:42:23 DEBUG    Hypervisor name is 'kvm'
Sun, 07 Aug 2011 10:42:23 DEBUG    parse_disk: returning {'format': None, 'bus': None, 'readOnly': False, 'volInstall': None, 'path': '/var/lib/libvirt/images/fd1440.img', 'device': 'floppy', 'volName': None, 'conn': <libvirt.virConnect instance at 0x1ae5908>, 'size': None, 'driverCache': None, 'shareable': False, 'sparse': True}
Sun, 07 Aug 2011 10:42:23 DEBUG    Path '/var/lib/libvirt/images' is target for pool 'default'. Creating volume 'fd1440.img'.
Sun, 07 Aug 2011 10:42:23 ERROR    Error with storage parameters: Size must be specified for non existent volume path '/var/lib/libvirt/images/fd1440.img'
Sun, 07 Aug 2011 10:42:23 DEBUG    Traceback (most recent call last):
   File "/usr/lib/pymodules/python2.6/virtinst/cli.py", line 480, in disk_prompt
     dev = VirtualDisk(**arg_dict)
   File "/usr/lib/pymodules/python2.6/virtinst/VirtualDisk.py", line 437, in __init__
     self.__validate_params()
   File "/usr/lib/pymodules/python2.6/virtinst/VirtualDisk.py", line 946, in __validate_params
     self.__check_if_path_managed()
   File "/usr/lib/pymodules/python2.6/virtinst/VirtualDisk.py", line 864, in __check_if_path_managed
     "existent volume path '%s'" % self.path))
ValueError: Size must be specified for non existent volume path '/var/lib/libvirt/images/fd1440.img'
root@Boomer:~#


t@Boomer:~# virsh --connect qemu:///system vol-list default
Name                 Path
-----------------------------------------
Donkey.img           /var/lib/libvirt/images/Donkey.img
fd.img               /var/lib/libvirt/images/fd.img
grub-floppy.img      /var/lib/libvirt/images/grub-floppy.img
root-rh-7.3.img      /var/lib/libvirt/images/root-rh-7.3.img
Win7.img             /var/lib/libvirt/images/Win7.img

root@Boomer:~#
root@Boomer:~#

In the virt-install example, you try to use volume fd1440.img, which doesn't
show up in the vol-list output, meaning virt-install thinks that file doesn't
exist, hence why it says you need to specify a path

What's the output of ls -l /var/lib/libvirt/images

If the file is actually there, you might need to refresh the pool with 'virsh
pool-refresh default'

It happens I have rebooted, and the problem wrt that particular disk image now does not occur.

The file did, in fact, exist; I've changed the way I pass data to my new guests and now do not use a floppy
image, so I used to commands I already showed you in order to recreate the problem.

Previously, I expanded the allocated size of the file using a dd command similar to this:
   dd if=/dev/zero of=/var/lib/libvirt/images/fd1440a.img seek=1 bs=2G count=0
   and the problem went away. Since that it now does not go away that way, I expect I must have done something
   else as well, lile restart libvirtd.

Since the message, "ValueError: Size must be specified for non existent volume path '/var/lib/libvirt/images/fd1440a.img'"
is clearly misleading because the ls command (which is the obvious way to check) reveals the file does exist.

I suggest that libvirtd (or whatever is the offending code) be changed to ask the filesystem whether the file exists
(stat() and/or access()) and that the error reporting be improved to more accurately reflect the actual problem.

Thanks for your time.

This sounds like you've not refreshed the storage pool after adding the
image? I leave the bug open for now since the error message is
misleading indeed.
 -- Guido

I certainly didn't, I don't recall any documentation to the effect that it's necessary.
Note that it's not required in this example:

# dd if=/dev/zero of=/tmp/fd1440.img count=2880
# /usr/bin/virt-install -n dummy --pxe --debug  -r 192 --arch=i686 --vcpus=1 --os-type=linux --os-variant=debiansqueeze  --disk path=/tmp/fd1440.img  >/tmp/vm.log 2>&1

What virtinst version is this? Current release should refresh the storage pool
if it doesn't find the referenced basename.

- Cole