Hmm. Ser ut til at det ikke finnes noen enkel løsning, det er browseren som bestemmer hvor mange som vises.
"If by drop down you mean the standard "select" html element, then there is nothing you can do about it.
Every browser renders it differently, (depending on position of the element) IE7 is showing ca. 30 items, FF2 ca. 20. You can set it's size attribut to higher number which will cause drop down to be rendered as listbox with the visible number of items you specified as size, but that way you loose drop down behavior and you probably don't want that.
AFAIK your only option would be to find custom coded script which uses either flash or javascript/dhtml to render it's own, customized dropdown."