I have never seen an argument for abortion based on size, so this is a complete red herring. In the spirit of which, I would point out that the vast majority of abortions happen when the foetus is smaller than a grain of rice, and with the same cerebral capacity.
A foetus is not a person at an early stage of development, just as an acorn is not a little tiny tree.
Leaving aside for one moment the question of whether the foetus is a human being, the importance of the location is that it is inside the body of the mother, whose status as a human being is not in question. What is more the foetus can cause harm, sometimes very serious harm to the mother. The only person who can permit this situation to continue is the mother. Forcing her to go through an unwanted pregnancy and risk harm is contrary to her human rights, rights to which the foetus is not entitled.
The point of dependency is that the foetus, and only the foetus, is totally dependent on one sole person. If you can find a way to remove the foetus from the womb and care for it some other way, then I would be content to ban abortion tomorrow. The aim of an abortion is to end the pregnancy. It is the pregnancy that is terminated. Put some church funds into find a way to do that without killing the foetus, and you can claim your Nobel prize.
Abortion is justified by the decision, upheld in courts and legislatures throughout the world, that the foetus is not a person. There are several reasons given why this is so., but roughly they boil down to two. Firstly, the foetus is not a person because it is not viable. It cannot survive outside the womb as an independent entity. The second reason given is that to be a person requires a degree of sentience, which is absent in the foetus. Personally, I find the first reason more persuasive. Neither reason is addressed by your ludicrous SLED acronym.