[Fedora-xen] Error in fedora quick start for Xen

[John Que]

Hello,

 I am following the instructions on Fedora Xen Quickstart page:

http://www.fedoraproject.org/wiki/FedoraXenQuickstart?highlight=%28xen%29

First thing, I want to say that the size for the fedora.img is
not enough now (I assume it was enough in the past)
because , after running (according to the instructions):
dd if=/dev/zero of=fedora.img bs=1M count=1 seek=1024
...
...

then running :
yum -c http://people.redhat.com/~katzj/yum-xen.conf --installroot=/mnt
-y groupinstall Base
gave me an error because of not enough space.

so I started the process whole over again from the beginning (and this took
more than an hour), this time allocating 2 giga:
dd if=/dev/zero of=fedora.img bs=1M count=1 seek=2048

So maybe this should be corrected in that web page.
I have a question:
Can the size of a file which was created thus
(dd if=/dev/zero of=fedora.img bs=1M count=1 seek=1024)
be changed ***without*** causing a damage to its contents?
In case there is such a way, how can it be done ?
Regards,
John