1. Make it in Photoshop.
2. Size always depends on how much content you want in it. I usually start off the width being around 1100px wide. Then I start from the top and work my way down to the bottom adding on what I need.
3. If you want it to be clickable its the same way as you'd do for a signature. Slice it up and link all the separate pics.
http://www.sythe.org/graphics-resources-discussion/1472743-tutorial-how-slice-link-any-image.html
4. C4D Is really only good for the header part of a thread design. It makes it stand out more and for the most part it's more visually appealing than none-3D text imo.
Need anything else? Just ask