There is no standard barrel racing arena size, but there are rules about course setup and distances within an arena. The setup within the arena depends on the association under which the event is held.
Most barrel racing associations specify expectations regarding the types of barrels to use as well as the course layout. For example, the Midwest Barrel Racing Association (MBRA) designates a cloverleaf pattern in which barrels are set 15 feet from the fence with a score line at 60 feet from the first and second barrels. The International Model Equine Hobbyists Association (IMEHA) Western Barrel Racing often uses an isosceles triangle formation in which the first and second barrels are set at least 18 feet from the fence.
Most associations indicate that the measurements should be in proportion to the available ring size and should allow room for the horses to make their turns.