Shaping rice for your sweet sushi treats is best down when the krispy mixture is still slightly warm. If it cools and firms up too much, warm it slightly in the microwave for easier forming.Link (Thanks, McAuliflower!)
Rolls: The rice base for the rolls is easily shaped when one finds the appropriate sized circular object to cut out cylinders of rice krispy pieces. The average sushi roll slice is just over 1 inch across. Many circular cookie cutter packs will come with a circle small enough to cut pieces for the rolls. When in doubt- make your piece small so that it is bite sized and can be popped into your mouth in one fell swoop.
Nigiri: To shape the rice base for sushi draped nigiri, I was fortunate to have a nigiri press (featured in the picture) on hand. It produces a piece that is a 1″ x 2″ rectangle with rounded corners, much like a pillow. Alternatively, this shape can be formed by hand.
See also Chocolate sushi
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.