A parcel of land is just some undefined quantity of land. It does not contain a precise amount. Parcel is the same as piece. You cannot know how many square feet it contains unless you qualify the size of the parcel of land.
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Bruce is correct, a parcel is a piece of something, or a portion of something. The WordReference dictionary says it is an 'extended' piece of land, while my old Oxford English Dictionary says it is a 'small' piece of land. Neither gives any specific dimensions.