You are asking me what size of dog crate? Obviously this is ment for chicks , the 3 week old would easily have room in a large crate .
Your not looking for a permanent solution here, just a run on the grass with safety .Why can’t you put them in your run? You can clicker train them all you...
I would not let them run free , as you said there’s dogs near by one mouthful and you’ve lost 4 chicks ☹️.
Get a dog crate take the plastic tray out so they’re on the grass. A fence won’t help because they will be flying over it soon.
With 36 it could go bad very fast! Hope for the best but...