Barn Size Needed for Alpacas
Each of our herds (ladies, guys, weanlings, sick, new herd...) each have their own pasture and shelter. For years, we had to make due with 12' x 24' lean-tos. The key for this type of building is to have at least 2 doors on two different walls. Some alpacas will stay out in a blizzard while others run to the barn with a single drop of rain. The doors allow you to shut out the wind / snow / rain without shutting your herd in or out. Alpacas are a bit claustrophobic and if they feel they will be shut in if they go inside, they may not go in.
We have found that 1 or 2 animals may use a 12' x 16' (192 sq. ft) three sided open face shelter with a 12 ft. roof to "loaf" in. In inclement weather, 12 to 16 alpacas will crowd in and 8 to 10 llamas. Given a choice, they will not crowd in more than one animal per 3' x 6' (18 sq. ft.) space. They crowd if the temperature is less than 20 F and windy, especially if the sun is not out to warm them. When sunny, many will remain outside as much as possible.
They will not use these sheds at a greater density than 1 animal per 6' x 8' space even in inclement weather when temps are above 90 F. A low roof does not allow effective cooling and ventilation.
Also, we have found that alpacas and llamas will preferentially stay within a three sided open faced barn that is 30' x 60' with a 20' roof rather than stay outdoors. Even on sunny days, where temps are above 30 F, they stay in the barn only coming out intermittently. They will easily stay in the barn during hot weather.
Perhaps this is an effect of 'individual space infringement'.
When they stay in the large barn, they stay anywhere from 2 to 10 feet away from each other. They tend to use 1/2 of the barn (the perimeter) for eating and sleeping and the central area for manure.
Thus, I think that social stress is based on the ceiling height and the depth of barn as much as anything. I would prefer a long, deep barn with a high ceiling.
That said, if you are planning on building your own barn, lay out your plans and then increase the size by 25%. You always need more space - for a sick animals, storage, shearing, machinery. The time to get a bigger building is before you build it. But, sometimes the budget just doesn't allow for it and you need to work on growing your outbuildings in the future.