Shaft Length and Head Lie are related and dependent upon your putting style and physical height -- the farther you stand away from the ball in your putting stance and the taller you are, the longer the shaft and the flatter the angle you will need. The best way to determine your specification is to compare your stance with what most would consider the ideal putting stance (pictured in the above graphic).
The ideal putting stance according to putting expert Geoff Mangum, is that your eyes are directly above the ball and your head is tilted such that your eyes are looking directly down at the ball, The usually puts your toes about 6 inches away from the ball. and makes the head lie angle about 75 degrees. This kind of stance according to Geoff facilitates a beneficial "pendulum stroke" instead of a detrimental "barn-door stroke".
The other thing you should consider is that the a shorter putter length might improve your putting stroke. To test this choke down on your putter and swing it back and forth at various places and try to feel at which place the stroke seems least waverly (if you have a putter that suffers from wobble, use something else as a pretend putter, maybe a cutoff broom stick or dowel rod). When you find the best place, put a pencil mark an inch or two above the heel of your upper hand - then measure from the bottom of the stick to the pencil mark and that is the length you should specify.
Or you can simply seek the aid of your local PGA Golf Pro at the venue you usually play golf.
Obviously, if your old golf putter does not fit your hand size and your putting style and your physical stature, it should be tossed in the trash heap and replaced with a better fitting golf putter. .
The Quantum Grip is round and the diameter is the same from top to bottom. It can be specified in one of three sizes: SMALL (1/8"),; Medium (8/8" = 1"), and LARGE (9/8"). If you are a lady or youth, the small size is probably best; if you are a man with average size hands, the medium is probably best; or if you are a man with large hand, then large is best. Although the trend nowadays seems to be toward larger grips, so what you specify is entirely up to you.