Thanks for all of the advice! We are building a shedrow, which is not a traditional indoors fully enclosed barn. It will have good air flow-I am putting grilled sliding windows at the back of every stall. The front of the stalls will be traditional grilled top and wood bottom. I am planning on figuring out some kind of fan system. I have several reasons for going with real stalls instead of the metal fence panel stalls that most people use out here. Most important reason, where we are building our house it is not uncommon for mountain lions to climb through the fence panels and drink out of the horses water, while the poor horses are out of their minds with fear. I am very reluctant to subject my horses to that. I want them to feel safe in their stalls. Secondly, the horses often kick right through the space in the bars of the fence panels when having a fight, and sometimes they catch a leg and hurt themselves. It doesn't happen often but still makes me nervous. My mare is a kicker. Thirdly, and least importantly, they might stay a little cleaner, which would be nice! I have had plenty of experience with the metal panel stalls, and I don't see how they are any better, they are just cheaper. We are putting in turn outs so it's not like they will be in stalls all day. Usually in summer they are let out in mornings and brought in about noon when the heat hits its peak. I have been inside shedrow stalls before in heat of summer-they are not any hotter than the fence panel stalls, and the horses inside of them were certainly not sweating, even when it was 110+F! There was a black percheron cross that was kept in one of these stalls. I visited him frequently in the heat of the day and I never found any sweat marks on him. He didn't even have fans! Granted, these stalls had air flow front and back, like I am planning on doing. I am sure a fully enclosed stall would be very hot!
That is some very good advice concerning shavings! I did not know that. I'll look around for some wheat straw... although my mom's mare eats just about anything lol hoping she doesn't eat that!
From what most of you are saying, it sounds like a 12x24 or 12x36 will suit our purposes just fine. Hay for four months would be plenty. Our horses eat about 3/4 of a bale a day. They are light, small horses, and very well fed. We are not underfeeding! In fact, my mom's mare is a little on the fat side, but she is a QH, and eats everything! The other three at the barn where I am at are lighter arab, half arab, and my mustang, who are on the sleeker side. They are all fed 3 times a day. I will probably be sticking to that schedule.
Thanks for all the advice everyone! You have been very helpful. I will post pictures next year after everything is built.