Final answer:
An additional circuit vent's size is determined by building codes, often ranging from 1⁄2-inch to 2 inches, depending on the fixtures served and the distance from the vent stack.
Step-by-step explanation:
The question pertains to the size of an additional circuit vent in plumbing systems. In such systems, an additional circuit vent should not necessarily be large; instead, it is usually sized according to the building code requirements which take into account the number of fixtures being served and the distance from the vent stack. For most residential applications, a 1⁄2-inch to 2-inch pipe diameter is typical, although local codes may vary. It is crucial to consult local building codes or a plumbing professional to determine the appropriate size for a specific application.