Is there any scientific results on the difficulty of picking elements from a list?
I am looking for a reasonably formal study of the impact of the number of elements in a list (e.g. a dropdown menu) on the time, effort, and loss of comfort for the user whose task it is to pick one element in the list.
It sounds like you're looking for Hick's Law: Increasing the number of choices increases the decision time logarithmically.
You might also want to look around at Nielsen Norman, who publish well-researched articles. Here's one on dropdown lists.