Derek,
hoping I'm not stealing the thread. There are many sorts of finials, from short squat ones (lid knobs) to tall and ridiculously thin ones.
As a guide;
It must suit the piece, e.g. Tall and thin is no good as a knob that will be regularly used, it will be too easily broken.
It must balance the piece, in the first instance look to the 'Rule of Thirds' for size/height, and
Consider the width dimensions in proportion to the main piece of work as well as the overall shape of the finial.
And most importantly, all rules can be broken, look at some of Andy Coate's postings and what he's used as lid finials, or the chilli or bamboo ones of Mark Sanger as a start.