A lot to decide before you build
You need to know how you want a horse kept. I want horses out 24/7 if possible...I built a fence divided south facing shed. I wish the shed was twice as deep but preparing a mound at least a foot higher than paddock ground to put it on was great. I wish the shed had electric for shed fans and a light for a quick look see at night. I have a waterer in the fence line in front of the shed...wish it was a square double sided Ritchie instead of a round Nelson. The shed serves two areas and that has been very valuable.
The horses are most miserable in the bug season. Heat and bugs makes them miserable...10 TIMES more miserable than winter weather. My barn has no insulation and is very cold in Minnesota winters BUT the air exchange is great and there is no stagnant ammonia saturated air at all even if I compost bed in the winter. I have 2 12x14 stalls one 12x12 and one 14x16 I have a never used washed stall I was talked into and a 12x12 tack. I groom but rarely bathe...never have...even with white horses. Know yourself. I got someone who I believed knew construction of horse facilities and mostly they did a good job... but they set electric lines under ground that have been damaged by gophers that are rampant in the area they and I are in...it will cost a lot to have the lines re placed but protected from gophers...the "wash" stall drains into the aisle and tack not down the drain. The concrete pad was well laid but not well slanted for water.
I have 1700lbs Irish Draughts and a itchy butt needs a sturdy wall and they can fold ordinary gates like accordions.
16 foot aisle...fabulous. I have one too small sliding aisle door at one end...pre-existing building modified for horses...and one high beautiful garage door that a fully loaded hay truck can drive into...hay barn addition...stores 6 months of hay easily and room for extra stuff.
I am on sand so I opted to not do more than class 5 the stalls thinking I would mat them later. I rarely use the stalls in the winter...horses do not want to come in. In the summer they want to be in all day out of the bugs in front of their fans(each stall has an outlet) the stalls drain fast and I never have to remove much bedding. If I had fans in the shed they would like that...if the sheds were darker and deeper thay would like that too.
I have 2 stalls that have dutch doors and the barn sides have a too low over hang(previous existing structure kind of too low for 17+hand horses) all the horses usually come and go from the barn out those doors. I was worried when I debated Dutch Doors that the ground heaving would stop up the doors in winter but the eave overhang keeps this from happening so I wish all the stalls had Dutch doors. The dutch tops are a PITA and often can't be connected to the bottoms.
HUGE safety issue if you have steel siding, never have unbacked steel. All walls need to be protected behind or in front of the steel to a generous kicking height. They WILL put a foot through the steel and it is a horrible injury we cause by being too cheap to line the walls...4 feet is common but I have to 6 feet and I lined the hay addition as well just in case I ever used it for stalls or as a shed.
In general I design so if something happened to me or I got to take a vacation someone could safely take care of the horses without going into the paddocks or touching a horse. I designed it so one woman could do the care of at least 7 horses or more. The horses would be safe and healthy and happy.