SRD:Size and Type (Creature Statistic)
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This line describes the creature’s size. A size modifier applies to the creature’s Armor Class (AC) and attack bonus, as well as to certain skills. A creature’s size also determines how far it can reach to make a melee attack and how much space it occupies in a fight (see Space/Reach).
The size and type line continues with the creature’s type. Type determines how magic affects a creature. Type determines certain features, such as Hit Dice size, base attack bonus, base saving throw bonuses, and skill points.