[palosanto]

What I am suggesting is that when you place a new stamp annotation, a popup will be displayed so you can change the size for just that annotation. It wouldn't make sense to leave the size indicator up, because it only applies to the annotation that was just created

One can use the handles to resize  stamp without changing the set size.  No need to display the size popup.  
However for text this doesn't have a useful effect. The handles dont resize the font.  It would be good to be able to resize the font similarly to the stamps.

Regarding the old cancel/save, I agree it seems a lot of work for the rare potential risk of losing the work, at least myself I usually do 3 or 4 annotations and exit annotation mode  so even if I lose power I don't see a huge effort in redoing them. It's not that you place annotations for hours usually ....
So I'm fine actually the way it is now.