When the download dialog is displayed, it should always be visible to the user, to ensure that the user realizes it is there. If the dialog is displayed in a small enough window, the user may not realize it is being displayed, and if the right keyboard sequence is carefully followed, they can end up running a downloaded executable. Additional social engineering steps are needed to ensure that the user presses the correct key sequence, without being able to show any relevant visual feedback, as the page cannot see that the keys are being pressed.
Multiple ways of generating too small windows existed in Opera.
Opera Software has released Opera 12.01 and Opera 11.66, where this issue has been fixed.
Thanks to Jordi Chancel for reporting this issue to Opera Software.