My traps average 10 bars long and I've been successful catching swarms with them. I build my top bars 17" long so I can swap them into a Lang with simple modifications. You want about 40 liters volume so depending on how big your top and bottom are will effect the length. Do the math, just goggle "Volume of a Trapezoid". Below is a link to my drop box with pics of them, I have modified the tops from telescopic to migratory since the pics were taken. As an experiment I built a trap yesterday like a stud space, 20"X4"X58". It holds 10 deep frames and I'm hoping to catch a swarm from a building where the ferals swarm to stud spaces.I'm going to build a top bar nuc to use as a swarm trap. Thinking about making it about 21 or so inches long and the same width & depth as my top bar hive. Any input would be appreciated.