A mimic can have almost any dimensions, but usually is not more than 10 feet long. A typical mimic has a volume of 150 cubic feet (5 feet by 5 feet by 6 feet) and weighs about 4,500 pounds.
Mimics speak Common.

A mimic often surprises an unsuspecting adventurer, lashing out with a heavy pseudopod. The creature does not necessarily fight to the death if it can succeed in extorting treasure or food from a party.
Adhesive (Ex): A mimic exudes a thick slime that acts as a powerful adhesive, holding fast any creatures or items that touch it. An adhesive-covered mimic automatically grapples any creature it hits with its slam attack. Opponents so grappled cannot get free while the mimic is alive without removing the adhesive first.
A weapon that strikes an adhesive-coated mimic is stuck fast unless the wielder succeeds on a DC 16 Reflex save. A successful DC 16 Strength check is needed to pry it off.
Strong alcohol dissolves the adhesive, but the mimic still can grapple normally. A mimic can dissolve its adhesive at will, and the substance breaks down 5 rounds after the creature dies.
Crush (Ex): A mimic deals 1d8+4 points of damage with a successful grapple check.
Mimic Shape (Ex): A mimic can assume the general shape of any object that fills roughly 150 cubic feet (5 feet by 5 feet by 6 feet), such as a massive chest, a stout bed, or a wide door frame. The creature cannot substantially alter its size, though. A mimic?s body is hard and has a rough texture, no matter what appearance it might present. Anyone who examines the mimic can detect the ruse with a successful Spot check opposed by the mimic?s Disguise check. Of course, by this time it is generally far too late.
Skills: A mimic has a +8 racial bonus on Disguise checks.

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System Reference Document -> List of Creatures (SRD)