Hi blauer,
I don't believe there is a way to set the dropdown list size--at least I can't find an exposed property that does this. However, you can put a dropdown control in a cell and essentially accomplish the same thing, and dropdown controls do allow you to set the dropdown list size. Simply assign the LinkedCell property to the cell it resides in so that when you select an item from the list it is automatically entered into the cell. Also, hold down the Alt key while you size the control to fit it into the cell and it will snap to the edges of the cell.
Damon