Parashas Masei · Passuk 35:16 · TT note 1
Chabad.org — English translation + Rashi
Torah Temimah points to Sanhedrin 76b (TT note on “בִּכְלִי בַרְזֶל”).
E-Daf — Sanhedrin 76b · Sefaria
English: It was taught: Rabbi says, it is revealed and known before Him who spoke and the world came into being that iron kills with any amount; therefore the Torah did not assign a measure to it.
Halakhic measure — iron weapons have no minimum size for liability.
Any iron implement, however small, that causes death renders the killer a murderer liable to death; the Torah gave no minimum measure for iron.