The TabStrip's vertical tabs have no default sizes, but have paddings, which are designed for text content. You can tweak them to your liking with CSS:
#YourTabStripID .k-tabstrip-items .k-link
There is no standard-compliant and cross-browser way to implement vertical text. Moreover, if the tab text is vertical, the tabs will become higher and fewer tabs will fit in a given vertical space. This will pretty much defeat the purpose of vertical tabs, which now allow to accommodate a larger (or unknown) number of tabs on a reasonable amount of space.
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