If you are using a skin like SkyBlue, which uses only CSS styling for the tabs (except for one arrow picture), you can do this:
.RadTabStrip_SkyBlue .rtsLevel1 .rtsLink,
.RadTabStrip_SkyBlue .rtsLevel1 .rtsTemplate
{
line-height: 40px !important;
}
Just change the 40px to whatever you want the height to be. Otherwise, if you're using something like Inox, keep in mind that the background images are css sprites (actually true in most cases, hence the great looking TabStrip skins), so if you're trying to change the height, you're going to get some extra sprite images visible. In that case, you would need to modify the height of the tab as well as customize the image to the new height you're looking for.