When its “windy” I use an old Enterprise jib but I have no idea of its size. To estimate the reefed area of a mainsail you could estimate by unrolling your mainsail and drawing a hypothetical line approx 90 – 100cm above the boom where the first reefing points normally go. Personally I would advocate fitting a reefing system as I find it not unusual to change sail area during a day, and the idea of carrying a second mainsail and changing over would be a pain. Its not difficult or expensive to do. But you may be a dedicated racer seting this lot up for your kids……….