A longer than wide PBF doesn't really gain you anythng. It does make the roll control harder to trim as the thing gets longer in comparison to the width. I've gone as far as 2/3 as wide as long.

You can gan better roll stability by going wider than the length... but roll rate suffers.

Square is about as good as they get.

The only place to really improve a PBF is in making the LE stiffer to allo higher top airspeed. With some of the softer coroplast, the LE is very flimsy and will start flapping at less than 2/3 throttle in level flight. Inserted wood in the LE and other things help... Anything that doesn't excessively increase weight and stiffens the LE is good.

I've got a 24X24 PBF that van handle a full throttle TT .46 pro in level flight. It tucks the nose as it speeds up, and can do an outside loop with neutral elevator.