The Scale Combat guys have some definitions for "scale" - and manage to handle it without fuss.

"3.2 Aircraft must resemble their full-scale counterparts. (...) All deviations from scale outline must be within ten (10) percent of each other and within ten (10) percent of scale, and retain scale shape and proportions. For example, scaling a model’s wingspan up 10% and the fuselage down 10% is not allowed. (...)

3.2.1 The Contest Director shall determine accuracy-of-scale by simple visual inspection at 15 feet and may, at his discretion, request from the pilot a three-view drawing or photo, to help in determining scale fidelity. The burden of proof of scale fidelity shall reside solely with the pilot of the aircraft."
...
http://www.rccombat.com/documents/20...%20v2009-1.pdf

(I think there are other organizations with detail differences in scale rules, but with the same general idea)