Ark as a permanent home?

I personally would go for a straight sided ark. You don't loose the space at the edges where the birds can't get in a triangular one.
The more room you can give the birds the better. I would have thought that for a couple of birds that size would be fine. A smaller ark is easier to move onto new pastures than one that is going to be too much effort, and giving them fresh ground to scratch about on will help to keep them occupied.

hi there kleinerp , I keep my 4 chickens in my backyard too. I have a small shed 4 ft by 4ft which I use to house the chickens. Only small modifications are needed, a pop hole in the side and roosting bars are all you need. Now my girls are laying I'm looking at putting in permanant nesting boxes but at the moment they are laying in a cardboard box with shavings in the bottom and are quite happy with this. I have built a run on the side of this 4ft by 10ft and 3ft high. The girls stay in this most of the time as we have a lot of urban foxes around who would happily gobble them all given the chance. I do let them out when I am around and they enjoy pottering around the garden, eating anything they can see growing
I don't think keeping chickens confined is cruel if they have enough room and enough things to occupy them. In my run I tie up fresh greens for them to peck and they have an under bed container full of chinchilla sand to dust bathe in. If they were not kept in the run they would most definately be eaten alive, if not by the foxes then the cats next door who have been brave enough to stalk them even when I am around

MAny thanks for those useful replies. I'll definitely get the biggest, roomiest accomodation I can, given the space available to me. I'm looking forward to getting a couple of hens within the next couple of months. Can anyone recommend any breeders in the North East?

cant help you with that but i agree with everyone else, if you let them out when your around and give them greens tied up so they have to jump to get them then they'll be fine, good luck.