Photoshop :: Render Clouds Of Large Canvas (6ft By 2 Ft)

I'm dealing with a massive canvas (6 foot by 2 foot). I want to render clouds on the background and have the same effect as it does on a image say about 640 by 480. However when i render clouds on a large resolution you lots of little clouds which looks horrible. The same goes with water paper. If you zoom into a small area you can see the clouds, but when viewing the whole image it doesn't look nice.

How do i get the big clouds on a big resolution rather than lots of little small clouds.