It will probably keep chickens in. If they really want to they can probably fly over it, but the trick is make them not want to. Chickens tend to not fly over things. A very common way for chickens to get out is to fly up to the top of a fence to perch and just hop down on the wrong side. Most full-sized fowl can easily fly up 6’ to roost or perch if they want to. But they need a place to land when they fly up. So don’t put anything solid like a top railing or even a wooden post top up where they can land on it. Have the wire itself be the top of the fence. That by itself will solve the majority of your problems of them getting out.
I’ve found that another common way for chickens to get out is when they get trapped against a fence in a fight or if a hen is trying to get away from an amorous rooster. The chicken is desperate to get away so it goes vertical and can easily come down on the wrong side of the fence. About the only time I see this is when I have a bunch of immature cockerels in the flock and they get in their fights. So try to keep them from getting trapped against a fence. 90 degree corners aren’t too bad but if you can, flatten the corners out more. That one is kind of obvious when you think about it, but another problem can be if the area is long and narrow as opposed to wide. I once configured my 48” high electric netting with a narrow corridor to get to a new area where it widened out. Cockerels were flying out on a regular basis. When I reconfigured it to a wider area, the escape stopped.
A 300 square feet area isn’t all that big, but the 6’ height should work pretty well. As long as you don’t give them sharp corners and don’t have anything solid up high it should keep you chickens in.
