my chook yard is just aviary mesh (12mmx12mm squares) and star pickets. When I put it up, I used 120cm wide aviary mesh. I had some 90 cm and also used that. I thought I'd have to join some to make it higher, but nobody's tried to get out.
I think it depends on the chooks and the circumstances (whether they are "fliers" and whether they might have a reason to clear the fence), but I've found that they stay in the yard with quite a low (by people standards) barrier.
EDIT: I have to revise what I said earlier on. It definitely depends on the chooks. I brought home some new ones today and one had no trouble "taking to the skies" and flying over the fence - BUT she had a hen house that she jumped onto before she cleared the fence AND she was trying to "escape" from me - she doesn't know me yet and she's very young, so hopefully she won't always be so "flighty". I expect that once she learns that I bring the food, she'll be more interested in that than escaping.
Edited by potato chip - 3/12/16 at 1:57am