I ordered 35" now with the option to extend it by 1".

That seems extremely long, but really you need to putt with what is most comfortable for you.

In perspective, I like my putters short. I'm 5'8" and I putt with either a 33" or 34" putter. You are 7" taller than I am, so that would mean up to a 40" putter. Yeah, that is tall!!! Really matters how you stand and where your arms fall.

Do a fitting if you can. That is obviously more difficult with a putter, not to mention that there are so many different ways to putt!

It really depends on the length of your arms or your preferred posture. Faldo used a 35” and I’ve read sometimes a 34”. He is 6’3”. Try a bunch of putter brands,styles and sizes.

Probably don't need to go any longer than 36" (max). I'm 6'5" and use 35.5". When I had back problems, I tried to order a Scotty at 37" and they wouldn't do it.

I saw somewhere a rule of thumb is to go by your pant inseam size while again it’s not exact it is a starting point. I am a little over 6’4 and I have a 37and a 36 (which is my pant inseam size) inch putter I play with depending how comfortable I am feeling over putts and how close to the ball I am.

If you're going to spend that much on a putter, you really should get fitted. Obviously go with whatever feels the most comfortable and inspires the most confidence, but a professional can assist you in finding that.

I would suggest seeing a certified fitter and let him/her determine your need by taking a look at your setup and stroke. They seem to know best.