How about building your own? you could build a timber frame(B&Q cut wood to the size you want, if you buy it from them), then mesh and coat it all with japlac
You could have it as big as you want it then and it would most likely be a lot cheaper than a metal bird cage, my guess would be around £30-60 to build your own
I'm currently building my own although its just a tiny bit bigger than what you want because mine will be 12ftx3ftx7ft (LxWxH)

lol!
The wood, mesh, screws, japlac and brackets have cost me £185 its best to shop around for the cheaper prices
But if you want a metal bird cage then how about
THIS?