Putter Length - How do you know?

a combo of height/arm length and comfortable stance. without a true fitting its just a feel thing. at 5'8" unless you have a real straight up stance id say 34" if not 33"
im 6'2" and use a 35"

It has to do with your weist’s distance from the ground. Any decent golf store can fit you fairly quickly.
Get the lie and loft checked too.

Nothing in golf is standard. Everything comes into play if you want to get exact about it. Height, wingspan, leg length, stance, body lean, eye position over the ball, etc.
Someone mentioned that they are 6'2" and use a 35" putter. That doesn't mean shite. I'm 6'2" and run a 33" putter and grip completely down on it because I have long arms and stand with my eyes directly above the ball with a good bit of hip flex.