Hi Sam,
Just picked up your response. If I have it correct you are concerned about the extra space the Archive will take up on your disk. Yes, you are correct, infact the archive will probably take up to 5-6GB space, but not to fear, you can delete this afterwards (after you are back to the main desktop that is).
I don't think there is an option not to archive, and I guess the reason is to do with the just in case you want to go back to previous state. But of course going back is not an option in your case!! To delete the archive once you have reached a working desktop, find the file in a special folder made by the install called 'previous systems'. A search may not find this (don't know why, it didn't in my case), but you can simply locate it by double clicking on the Macintosh HD icon on your desktop and it will be in the first sreen that appears.
Heres the tricky part that I found, and thats actually deleting it. You would think that just dragging it to the trash should do it, but it didn't work that way for me. It started deleting, then kept giving me a message that certain files were locked, and so I could not delete. I have to admit that I used a program which I have called Macpilot which gave me the option to unlock the folder and contents, and it deleted first time after that. I haven't tried this but I believe you can unlock either by highlighting the folder, going to file in the finder selection at top of screen, and select 'get info', with which you may be able to uncheck the locked option for the file. The other way (I believe) is to hold down the shift-option keys while emptying the trash folder.
Once you have deleted the archive you should have exactly the same amount of disk space left that you had before.
Hope this was the answer you are looking for.
Timberman.