I do a scaled down version of "Nordic walking", or the American version using "trekking poles". To start with, I only use one stick, usually on the side opposite my not so strong knee (the strong knee is a replacement from several years ago). I do not use it to go faster or use more energy and burn up more calories...I use it to give an extra push going up hills, and/or to add stability on rough terrain.
And this fits in with your question about the size of sticks...Nordic or trekking poles are relatively short, with the handle being at the top, at about 40+ inches from the bottom. They are usually adjustable to fit various heights and stride lengths. On my sticks I leave a bulge from the whorls which fit me at 41 inches. This allows me to adjust my grip between firm on the push and relaxed on the forward swing of the stick. So my "45 Magnum Carbine" hiking stick works well.
I modified the technique I learned from training sessions with Robert Sweetgall, a man who has walked across the USA several times, and he designed and now sells trekking poles. http://www.goyaasma.com/
Now, for the real twist...I always have a stick, but I don't always use it! Don't use it if you don't need it...carry it horizontally in a relaxed grip at it's balance point on smooth and easy grades. When I was doing physical therapy after my knee replacement the therapist would not let me hang on or even touch the side rails on the treadmill...she wanted my body to use all of the many sensors it has to determine where I was in space and keep my balance without a crutch.
Attached is a picture of Nordic walking...I don't have the longer stride, and I usually plant the stick even with the heel of the opposite leg. And that placement at an instant before the foot strikes eases the impact on the knee.
And another picture of my carbines with "handles".