I build my swarm traps as a bit of a compromise, between what the bees might think is best and what I can handle carrying through the woods, up and down ladders etc..
A Ten frame deep size is reportedly optimal, but a lot depends on what you can handle. ( Remember, it will be full of bees and weigh more when you take it down. )
With little time on my hands, I pulled a ten frame deep sized box out of a tree in the dark.. It landed on my head just about an eyeblink after my head hit the ground, I still have the scar on the side of my head to prove it. Then With the bees REALLY upset.. I had to carry it across the crick, through the woods, down the tracks to my truck... I left the ladder and had to go back a couple days later to get it.
Since then i have been a little more stingy on box size/weight, as well as how easy it was to get at the box. Back then, I was sure a box 12 feet up was the only way it would work.. Now I dont put them any higher that I can reach.. Usually while standing on the tailgate of my truck.