I am wanting to make a standing desk that is 40" tall. Would these work with legs that long?

They should work. I used 33" pieces for the legs on the sawhorse.

very unstable , no , you can buy legs already made to dowhat you want , do you want to be able to fold the legs up for storage ? other wise use premade legs for a desk

They're only 29" high. Add a 2x, and you're at 30.5". Even if you gave it a thick and heavy top made of 4x lumber, you're stuck at 34". Take a look at the review photos, and you'll see a desk that someone fashioned from these horses. I don't think these would work very well for adding extensions to the legs. I'm not sure I'd want a standing desk that had legs that fold up like this. You'll be much better off to build the legs from lumber or metal. These sawhorses would be quite helpful in helping you *build* your desk, though!