The following select element has a size of 4, and the parent has a padding of (clockwise from top) 2em, 1em, 0em, and 5em. You should only be able to see four options ("opt x" lines) at a time.
For some odd reason, the select is 'inheriting' the padding!
Here is another, with the parent having padding 0em, 5em, 5em, 0em, and the select having size=2:
This is bug 13562.
See also: