Tailoring Your Staff To Suit You
The general rule is to pick a staff that is somewhere between shoulder and chin height. But then it is what is comfortable for you and your personal choice of top.
Going for a thumb top, it will not be the same as a Wood Spirit Carved top with a grip below, or a minimal with the grip in close order. So it's an arbitrary decision which grip you will use most, remembering the Thumb Top is mostly used for watching daffodils.
Then, of course, one could do as I do and use mine for steadying the camera. I'll stop now; you get the idea. Give me a measurement, I'll ask a few questions and then trim to suit.