I think it's Michael in the coffin. Let's forget about the size for the moment...you know, the makers of the show may have just got hold of a coffin and not even noticed the size, and we may be theorising for nothing
ANYWAY...as Roco pointed out in another thread, we know the 'oceanic six' are known as celebrities of sorts, so i think if one of them was dead the death would be more publicised. There would be more than a small mention in the paper, and there would likely be journalists and all sorts. My theory is that Michael came back with Walt when Ben let him go home, but he didnt tell anyone about what had happened to him. Remember Ben, "You won't tell anyone about this. Or everyone will find out what you did". So Michael changed his name, hence the different name in the newspaper. Then for some reason, probably killing himself with guilt or something, he died. He has a teenage son, so that fits, and also no one would go to his funeral because everyone who knew 'Michael' thinks he is dead. Also, someone mentioned before that the funeral parlour was in a black community.
As far as i am aware, everything fits...unless i have missed something?
So yeah, i'm going with Michael for the moment, although i definately see how Ben and John could be possible too...especially Ben.