The first thing that comes to mind when I see big curls are the Caruso steam rollers. It's an amazing tool for generating large, healthy looking curly hair. The next two tutorials will show you how. Click on image for video.
YouTube: Miix Mii
YouTube: Taren Guy
Can you believe that these amazingly gorgeous curls were created using a flat iron? Click the image to find out how.
YouTube: Big Hair Bettyy
Here's a slightly curlier version using flexi rods and a pin curl set
YouTube: Miix Mii
These are just a few ideas. You can also blow out the roots after a roller set to maintain volume while breaking up the curls. Another idea could be to use a large barrel curling iron. If you are looking for volume, make sure to "fluff" out the roots.
Hope this helps. Have fun. Stay inspired.