A larger loft is a good idea although Birmingham rollers can be flown out of a kit box slightly larger than what you mentioned. Please understand that "free ranging" pigeons does not work. There are just too many hawks for the pigeons to survive. My birds are whistle trained to return to the loft, and I still lose a high percentage to the hawks. Peregrine falcons, Coopers hawks, and sharp shins are all bird eaters and will camp out in the vicinity of your loft. Redtails although not "bird eaters" by choice are opportunistic and will take any birds that land in the trees around your loft. I lockdown from Sept. to March and still lose way too many birds. Good luck in your endeavors.