I found this one: http://www.coolest-birthday-cakes.com/coolest-mm-character-cake-10.html
Explains how she did it, (2) 9" cakes, iced and piped, and Mr. Potato Head arms and feet (these could be made in gumpast)
Or this one: http://www.coolest-birthday-cakes.com/homemade_cake_recipe.html#c2
Says she used a 9" cake pan, but it also looks like half a ball cake pan on top, which could be done in cake, if you need a lot of servings, or rice krispies if you don't.
There's another one, but the picture link (to the page that describes it and shows a big picture) is bad, it looks like a ball-shaped pan, covered in fondant and decorated like the green m&M with fondant accessories. Here is the main page with the small pic: http://www.coolest-birthday-cakes.com/homemade_cake_recipe.html
It's pic 15.
Edit: as I went back to check my message, the ad over here ----> was the Red M&M with a "Mom" tattoo on
Thought it was a little ironic.