OK, this is a good one. Answers in the comments section.

Q: How do you change the visible number of options in a select box when it drops down, not rendered in the DOM, but when it drops down  ?

I've come to the conclusion you can't because of default parameters specified in the Gecko rendering engine. I know that a <select> element has a "size" attribute which you can access through Javascript with {oSelect}.size or use directly in your HTML with something like <select size="25"></select>, but that changes the number of options visible when the DOM is rendered and not on the drop down event.

For once I think IE and the Trident engine has an advantage over FF and Gecko. IE naturally defaults to display the TOTAL number of <option> items on drop down where as FF is limited to 20, then you have to scroll.

I spent some time looking through the MDC reference manuals, for an interface, some sort of API reference, but the HTML Select Element Interface seems to be limited to the usual foray of methods and properties.

I thought maybe with the introduction of Gecko 1.9 (FF3) and Javascript 1.8 that there might be some interface for changing the default, but wading through documentation there appears not.

So my answer to the question, at the moment is no, but I will be making a suggestion to Mozilla !