Will these stretch the shaft of a boot and if so, how much. I have some cowboy boots that I need to have the shaft stretched to fit my calves.

These were junk! Sent them back, they would not fit firmly in the boot-nor stretch them. Went with another make.

Will these work as shaft stretchers on tight leather knee high boots ?

No. There wouldn't be enough tension against the side of the boots.

How do they work on Cowboy Boots?

I use mine for shoes only, and they do work very well. I assume they would wok well with boots also.,

Will this product support tall boots that are over your knee？

These are only good for knee-high boots, unless your over-the-knees are very stiff at the top. It might be possible to roll up a thin piece of cardboard or plastic and make a tubular upper extension that might hold up the top end of over-the-knees. You'd have to experiment. But just a suggestion, I hang all my boots with boot-hanger hooks instead of using a tree. Works great for any height boot.