How to make a scroll bar in the options values of a SELECT

Hi !

Please excuse my poor english.

I explain my problem :

When you have a SELECT in a form, you have a list of OPTIONS. When you click on the Select, this list appears in a box. A scrolling bar appears automatically when the number of options is greater than 20. But I would like to have this scrollbar with 5 options only.

And I can't find anywhere how to do it !

Thanks a lot

Pierre.