I did talk about ergonomics for benches in the old frankenbench plans, once upon a website dark and dr…green.
The short form:
Find a comfortable chair, sit in it so your feet are flat, and back as straight as you can. Sit up straight. Really.
Then have somebody measure from your armpit down to the floor. A pretty good desk height is armpit minus 2-3 inches.
Personally, I have a bunch of different benches, some standing, some sitting. Sitting for precision, standing for soldering and general mucking about. I’ve found it helpful to move around. Keeps the blood flowing. And for larger scale stuff, it makes it easier to get full range of motion on my arms, and get full power for those motions.
As far as buying adjustable ones, no, I don’t know of anybody making any. (Or certainly not that are rigid enough) I’ve got a back burner project to address this, coming eventually to KC.com, but not any time soon.
You’ll have to tweak your own. You can take a short bench and put it up on blocks to get it to the ‘right’ height. That’s probably the easiest thing to do.