This is two separate ideas: Restrict to predefined resolutions, and confirm a resolution change.
The resolution change confirmation is interesting. We can mistype the resolution, mispick from predefined resolutions, misremember our monitor resolution, ..., and such confirmation will catch it all. The classic Windows confirmation dialog accepts both mouse and keyboard input, and defaults after 10-15 seconds to rejection of the new resolution. It's nice, but it's hard to say how urgent this is. The gut feeling is that it's not urgent even though Silken would have needed it. On the other hand, if it prevents permanent lock-out, hmm.
I'm not so hot about guessing predefined resolutions and offering only those. If you catch bad resolutions with the confirmation dialog, why introduce another limit? It would also go contrary to dragging borders (left to be researched).
If you want it purely for convenience, we can add a fixed list next to the unconstrained number entry field. This has its own merits. We'd merely invest a lot of menu space and development into an option that you rarely change.
-- Simon