RadButton default state is built with sprite images for background in order to have gradients and rounded conveners in all browsers. RadButton custom height tutorial
explains how to create different custom heights.
Also at the Button Demo Page
you could check other button types - Link and Skinned Button, which are fully CSS based and do not use background images - that means that the button could have any height without any visual glitches. Note that Skinned Button type uses CSS3 for gradients and rounded corners and it will work in modern browsers only.
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