I'm so glad I found this thread. I too was getting very frustrated that my 1Password windows couldn't be resized. I must have hit F11 at some point without realizing it, and this state even persists across reboots.
Confusingly, even though the window can't be resized, it still has a restore button in the title bar, which just doesn't work. I think this violates Windows interface guidelines -- full screen windows aren't supposed to have the same elements as a maximized window, exactly to minimize this confusion.
Glad I found this fix, but I feel bad for the next guy who, like me, wastes a couple of hours before stumbling upon this community post!