You might find plans a la internet, perhaps
www.fiascofarm.com might have some. You can never have it big enough really, goats love their house, they just want to hang out near it or in it all the time. You can create different levels just putting in straw bales and create flexible internal arrangements with pallets so you can change them at a later date, or you can't go wrong investing in some flake gates - take a visit to your local fencing / gates yard and see what there is, also fleabay is good or gumtree has surprises sometimes!
Actually if you're handy, the fiasco farm website has the drawings for a goat crush which is a real must for when you want to trim their feet which you should do about every month or so and for doing anything else you need to hold them still for. Many of us here on tas have made it and its invaluable. It can be made from pallets. Fencing/gates suppliers also do something called a lamb adoptor which is similar and just costs less than £20 which would do a similar thing though not quite as handy as the fiasco farm one.
If you have a rain water gutter on your goat house then feed it into a water butt, that way you can have instant fresh water for the goats each day near at hand.
Good luck with your goats and get some photos on when they arrive!