That is still a good size.
To seal it up from predators there are a few options.
First know what predators you may be dealing with. Some people have everything from weasels to snakes and bears.
What part of the country are you in? You said barn so I assume you are not in town.
Pretty much any hole or crack 1/2" or bigger will need hardware cloth or more wood to keep weasels , rats and snakes out.
You will need ventilation no matter where you are. There are really good articles written by some really knowledgeable people here.
https://www.backyardchickens.com/articles/
Also tons of coop ideas in the coop pages.
I look forward to seeing your progression.
Be sure to post pics and ask questions on anything that gets tricky. There are always people ready to help.
Oh one thing that occurs to me......if you were to lay it down.....the boards that run horizontal would be vertical...... That would make it easy to give it a board and batton look. Here are a couple short videos if you want to see what that looks like while closing gaps.
Ok one video and one page showing what it looks like.
https://cabindiy.com/how-to/board-and-batten-siding/
Wishing you happy building!