What os/widget interface are you using? Have you tried TabWidth?
Its doesn't work for me, since as far as I can see, the nbcTabsSizeable capability is only implemented for Windows and QT. (Im using GTK)
Apart from that, you either have to "roll your own" control, or use another control with TNoteBook, or find a custom TPageControl that supports tab resizing, or (if you using GTK) try to implement nbcTabsSizeable.