I found this question, and while listboxes are cousins of the dropdown box, I think there are unique properties of combo boxes which require a seperate question.
This question on stackoverflow talks about the different rules for different browsers:
(a) the height needed to show all entries, or (b) the height needed to show (x) entries (with scrollbars to see remaining) (20 in Firefox & Chrome, 30 in IE6,7,8, 16 for Opera 10, 14 for Opera 11, 22 for Safari 4, 18 for Safari 5, 11 in IE5.0,5.5) (c) In IE, if there are no options, a stupidly high list of 11 blanks entries.
And this can be seen in the following screenshots:
Another possibility is to set the height of the drop down to span the whole viewport (and add scrollbars as necessary):
Can anyone provide research/evidence/experience as to optimal heights of comboboxes? There seems to be quite a large difference if we look at the rules provided by browser vendors.