My daughter rode a 2sp automarix hub Java 14" foldie for a kids triathlon.
Beat a fair number of kids on fancy racers even...
But for the adult field (esp if its OD), I'd say that you need at least a 406 wheeled bike.
My wife did the sprint distance and had folks asking her if her Pocket Rocket was motorized.


I think the best approach is to come up with the dimension that is allowed in the overhead bin.
Once you establish that, you can look around at folding bike specs for their folded size.
You might still end up with a Brompton, but then you'd know that its the only option for the compartment size.