I'm sorry, but in no universe is Shin two feet wide. I think you might have forgotten I'm over 6ft 3" myself with broad shoulders, I used myself to judge. If he happened to have been standing in the widest stance of all time then by all means that might be possible, but that would simply mean he should have been stabbed in the feet. However he was in the act of stepping. Plus, it's really irrelevant how wide he is at the top, lower bodies are slender, and that's all the spikes needed.

If I had to judge, Shin would probably be about a foot and a half wide if he was in the act of stepping, presuming he has anything close to a normal human body type. 

That being said, this is all completely ignoring the fact these spikes of rock are flying at roughly twice the speed of an olympic thrown javelin.