For IMAA (International Miniature Aircraft Association) sanctioned events here in the USA, the rule is 60" for multiwing and 80" for single wing. Different organizations and countries may have different standards.
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International Miniature Aircraft Association sanctioned events are alike the world over. Hence the name and purpose. "Giant" scale is defined as 60" Bipe, 80" Mono (whether it is a "scale model" of a "real" aircraft or no), smaller if it is at least quarter-scale (like the Shoestring pylon racer, for example) of a man-carrying plane that flew at least once.