D20:Size and Type Creature Statistic

====Size and Type====
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, below).
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.

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System Reference Document -> Reading Creature Entries (SRD)