Thanks for the shout-outs guys! If we're talking "foam" as a general material, then Peter's 30ft plane has me beat. Below that would be my 24ft C-47, which was also foam, but not "foamboard".
If we're restricting the description to Adams-style "foamboard", then - to my knowledge - the B-52 we built at Flite Fest 2019 is the largest. 14ft wingspan, 12.5ft long and ~32lbs.