In the interest of standardization, the RadComboBox allows a custom CSS clas to be specified for the dropdown, via the DropDownCssClass property (as if you probably couldn’t figure that one out :-D).  In an application I’m working on, we have a set of pre-built CSS classes of varying sizes, and we apply styles to the drop down to be consistent.  However, after applying the style, it didn’t quite seem to take affect.  This is because the CSS for the RadCombo looked something like the following:

<div class=”.. Telerik CSS Classes .. MyDropDownClass”>
    <div class=”rcbScroll rcbSlide” style=”… height:380px;”>

I don’t know where it got the 380 px, but then changing my CSS to:

.MyDropDownClass
{
    width: 250px;
    max-height:300px;
}

.MyDropDownClass > rcbScroll
{
    max-height:300px;
}

Did the trick for me.  May need to adjust slightly, as I haven’t fully tested cross browser.