Based on the recent interview I did with the fantastic website Putter Zone.com, I decided to present some visual clues that help me when I fit people for putters. I usually start with general information, like what the knuckles-to- ground length is when someone is standing tall with arms hanging to their side and making soft fists. Anything longer than 31 inches makes me lean towards a 35 inch putter or longer, and anything shorter than 31 leans me toward a 34 inch putter or shorter.
In the hundreds of fittings that I have done over the years, standard length for a putter to me should be 34.5 inches. I have no science to back me up, just instincts. As I said in the interview, if I am going to err, it will be on the long side because I want people taller, more athletic and relaxed. Aside from static measurements, like the aforementioned, I also consider torso length vs. leg length and sleeve length. The measurements give me a base with which to start. As all fitters know, there can be a difference between static measurements and dynamic ones. So, off to the putting green we go armed with options.
I am 5' 10" with knuckles 32 inches from the floor. My pants are 32 inseam and my sleeves are 32. My golf clubs are 1/4 inch over Titleist standard length. My putter is 35 inches and 69 degrees. I will touch on lie angle later.
In the first picture, I have a 34 inch putter.
What I first see is hands and arms too far underneath the chest, spine angle too tilted (40 degrees) and eyes too far over the ball. The angles I have created look to me like I am going to take the putter too straight back, maybe a little outside, and probably shut because my arms are stuck.
In this picture, I have a 36 inch putter.
While my posture looks better, my hands are just barely too far away from me, my eyes are a little too far inside the line and I am just a tad too tall (50 degrees.) My putter will most likely make too big of an arc, and I will have difficulty seeing the line. However, one of the reasons I sometimes err on the long side, especially when someone is coming from a severely bent over posture, is that I want to exaggerate the feeling of being up with freedom of movement. After a little while, I will back that person down to a properly sized putter.
Now, the correct fit. 
My eyes are on the heel of the putter, my hands are hanging perfectly underneath my shoulders and my spine angle is a comfortable, athletic 45 degrees. I am free to move and the putter will swing on the correct arc because it is designed to. All I need from here is a good grip, good alignment and then get out of the way!
While I am looking at length, I am also checking the lie angle. The putter needs to fit the person, not vice versa. Because of the grip I advocate, with the left hand heel pad resting on top of the putter grip, my lie angle fits tend to be a little flatter than industry standards of 71-72 degrees. What I have someone do is stand up straight, hold the arms out in front with the elbows resting on the rib cage and the putter parallel to the ground. Now, simply drop the putter down and let the body respond so the spine is around 45 degrees. If the toe is up, the putter is too upright and if the heel is up, too flat. Should be easy enough.
To get the correct grip size, please see my earlier post on how I believe you should hold the club and why. I hope this gives you a good idea of what to look for. I have people from around the world send me pictures to see if I like how their putters fit. I am happy to give it a go, so please feel free to do the same. Good putting is only a good setup away.