The way the current ComboBox is made, we use a lot of sprite images. Simply changing the "height" property will not do anything.
We do have and help topic on changing the height of the combobox -- http://www.telerik.com/help/aspnet-ajax/combobox-appearance-change-height-input-element.html
The article is a bit dated, with regards to a recent change in the Combo, but is still quite relevant.
the Telerik team
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