Just did this last November, and the dimensions are 20x10, so smaller.
I wanted to be able to move it, so didn't make it as deep as really wanted.
Are you planning on this being permanent, or are you going to ever move it?
For the wood that touches ground, you need to get pressure treated lumber, of course.
If you are wanting 24 foot long, make sure your lumber yard has 5x5's that long, or whatever type you want.
And one thing I did not take into consideration? Is that if I wanted interior size of stalls to be 10x10? (Which I did) I should have gotten 12 foot side boards, so when joined, the interior would be size wanted. AND the front/back boards should have been 22 feet long, so could cut down all four bottom boards to leave me inside of 10x10.
I found it actually cheaper to use metal on sides too, no painting, and no eating of it by horses. I got roll of insulating deal, that was silver one side/white other, that is too keep heat out better, cold out too. Ended up with whole roll when all needed was just the top length/width.
So put it on inside of stalls too, as well as roof. Covered the inside of it with plywood 8 feet high.
And where you are wanting to have two stalls? Put another 12 foot or so length in middle to divide. Figure out what you will be dividing with, boards, gate, solid wall, if you are going to make two stalls that is.
I found when I had smaller one made of wood? One horse would keep other out, with two separate stalls? Each can get in. And of course with Kola, I have to be able to keep him separate.
I made the wall in back 8 foot high, and the top is 12 feet, with overhang.
I will try to post picture...but don't know if can.
Honestly, after all the headache AND the cost of materials and labor? I would have come out much cheaper by having lumber yard build it and bring it to me.
Horses make me a better person.