Rashi on Numbers 35:17/רש"י על במדבר ל״ה:י״ז

1באבן יד [AND IF HE SMOTE HIM] WITH A STONE IN THE HAND (more lit., with a stone of the hand) — one which has the size of a full hand (i.e., large enough to fill a hand).

2אשר ימות בה signifies: which has a size sufficient to cause death, as the Targum understands it. Because it has been stated, (Exodus 21:18) “If one man smote another with a stone” but it assigned no size for it, one might think that it means, whatever size it may be! Therefore it states here, “by which one may die” (Sifrei Bamidbar 160:3).