The first stone was probably quite small, but useful for chucking at a speaker he disagreed with. This could have been followed by someone else finding a larger stone to throw. Then the mob, who were always willing to join in any uprising, all started hurling the stones. The first few would have grounded Stephen and then as the assault went on the life would have been battered out of him.
The crowd probably didn’t set out intending to kill the first martyr but one thing led to another. The message that they were hearing from Stephen was not to their liking. It reminded them of the man Jesus who they had also managed to have put to death. Big bad things start in a small way.