I have the exact same cam as you, have upgraded it about 3 month ago to 1.00f in order to be
able to install CHDK and have experienced no problems whatsoever, that is, I did not notice
any change in camera behaviour (except of course that it reports a different version)
This update is AFAIK from Canon, it just has never been publicly released.
As with all firmware updates, make sure that your power (batteries) does not run out during
the update (use a fresh set)
The reason for the size is that the entire firmware will be replaced.
I hope that gives you the info you need.
wim
edit: spelling