Are your rabbit cages going to be on the ground or hanging?
- If hanging csges are used then the roof and leg structure must be strong enough to support the weight of the cages + adults + kits + accessories (water, food, nest box) + snow and the legs must not warp or splay out from the weight.
- if on the ground then walls may be needs to prevent driving rain and drifting snow from getting into food bowls or nest boxes or to offer shade from the morning and evening sun
To figure out the size of carport you'll need you must calculate how many cages you plan to have
-# buck cages
-# doe and kits cages
-# grow out cages (depending on butchering age you may need one for the boys and a separate one for girls)
-# replacement doe cages
Extra space will be needed if you are going to keep any rabbit supplies in the carport - pellets, hay, bedding, unused nestboxes, medical kit, gloves, tattoo machine etc... And these need to be safe from the weather as well.
Heat is deadlier to rabbits than cold so direct sunlight is best avoided and some sort of shade or insulation may be needed to prevent a metal carport from turning into an oven.