Hi,
I don't think we have such a thing actually. I think the solution at the moment will be to manually set the size, which is complicated by the fact that the client interface to do this also includes the height of the editbox portion of the control.
So basically, what you're looking at is obtain the height from each of the attached items and sum the results to get the required height of the rendering area part (call this items_height). Then you need to obtain the editbox child control and ask it how high it is (call this edit_height). Then you need to discover how much of the height of the drop-list is taken up by imagery - one way to do this would be to query the height of the drop-list, then get the height of the rendering area via the attached looknfeel (taking into account the possibility of a horizontal scrollbar, so either named area "ItemRenderingArea" or "ItemRenderingAreaHScroll"), subtract the latter from the former and you get the height of the list imagery (call this imagery_height). The final size of your Combobox would then be items_height + edit_height + imagery_height. This assumes a traditional layout within the looknfeel - if your list part is offset vertically at all, you'll likely have to factor that in also (which is the difference between the bottom of the editbox and the top of the droplist).
So, yeah. That's a bit of a crappy way to have to do this, but I think it's the only way at the moment
CE.
Edit: re-reading your post, I'm not sure if you're using the full Combobox or just the DropList. Please clarify, as it makes some of the above a little simpler