You need to set the http://msdn.microsoft.com/en-us/library/system.windows.forms.combobox.integralheight.aspx to false when you setup your control (it defaults to true). From MSDN:
When this property is set to true, the
control automatically resizes to
ensure that an item is not partially
displayed. If you want to maintain the
original size of the ComboBox based on
the space requirements of your form,
set this property to false.
Add this line before you add the combobox to the Controls collection:
cb.IntegralHeight = false;
Reference - http://stackoverflow.com/questions/3868907/combobox-maxdopdownitems-is-not-working-when-adding-items-using-the-click-event