Hi LP,
the small test hedge we made probably did drop about a foot, as the brash when put in was green, but we topped that bit up around a month ago, the width inside the hedge is approx between 18" - 24" it changes sizes as it moves along,lol
yes it deffo is dog proof, we had our jack russell with us, and she had to keep running around, unable to find a way through, although we do use hazel rods woven along the bottom to a height of about 8" as it holds everything together,
for areas that need some sort of boundary, its a very quick fix, I'm sure it'll need a yearly fix, and so I'm told, becomes a haven for rabbits, which I'm happy with if true