what arf trainer has the largest wing span?
Well the Sig Kadet Senior ARF has a w/s of 80" so I would say this is probably it......but if you consider the Bruce Tharpe Flying King to be a trainer, then it would win with a w/s of 80.5".
Oops....I just re-read your subject line and you say ARF trainer, which of course the Flying King is not.