Can't directly answer your questions, I don't have Leopard yet but I did go to the release event and went to hear some demos. I asked if the Time Machine backups were, like the persistent rumours said, non-bootable and it is true so SuperDuper! is not obsolete by any means.
I asked the presenter if you can manually select which type of files you want Time Machine to handle and sadly, with Time Machine, it's all or nothing i.e. you either do a full backup of everything or you don't.
I also asked if you could delete certain backups of files from a certain date and later and you cannot: you either keep the whole backups of that file or you delete them all.
The suggested size: there isn't one according to the presenter. Time Machine backs up till there is space, then it starts deleting the oldest backups to make room for the newer ones. I would guess the usual logic will work in this case: at the very least, twice the size of your internal hard drive for the external.
And it was mentioned that the external used by Time Machine must be used only for Time Machine: it cannot share any other files or data on it, especially not with a Tiger install/backup for instance. It wasn't clear to me if partitions were ok or not.