What there standard sizes for stair steps/treads?

Question

I am asking because the treads I am replacing are 44" long (three of them) and the longest I have seen is 43". I think this type is called open at one end.

Answer 1

There is no standard length, just IRC code on the height of rise and depth of run on the stairs, even that has a lot of variances.

I believe what you have is an "open riser", although open riser can mean the absence of a riser board as well. The tread you have sketched appears to be a "left hand return tread" without the return piece added.