Just as big and as well insulated as you can find. Get a cover as well. I'm not great on outdoor bun homes as mine's an indoor bun, but there are a lot of smaller companies out there that make them. Have you tried a local carpenter?
but, YY I'd look for a local carpenter to make one. Advantages are, they can do alot of the assembly in situ to save you having to transport a heavy cage. You can be specific about the size you want and the materials, especially fastening/locks and the wire mesh. Some of the ones I've seen as really feeble.
DD can be embarassingly judgey "That's not even big enough for my boars let alone a rabbit, You won't manage two hops in there. And LOOK at that mesh. A fox would be in there in minutes"
(She's 11yo, she'll get worse)
They can advice you on what petsafe treatments for the wood too,
I find the Rabbit Hutch Warehouse excellent quality and very helpful and friendly, they do charge more for delivery depending on your distance from Bolton where they are based, but worth it in my opinion. www.rabbithutchwarehouse.co.uk/