in Japan. It's about 3 times the size of a regular maki (which is sometimes called a hosomaki (thin roll)), or about 2 inches / 5 cm in diameter. The traditional fillings include kanpyo (stewed dried gourd strips), tamagoyaki (slightly sweet omelette), sakura denbu (slightly sweet fish flakes dyed pink), cucumber and grilled anago (sea eel) They are hard to eat in one bite. One portion is usually at most, 1 roll (it's a lot of rice, besides the fillings), or you might get 2-3 slices as part of a sushi set.
A variation on futomaki is the kazarimaki (decorative roll), where the slices show various decorative designs.
(people get pretty creative with the designs)
One more variation on the fat sushi roll is the ehoumaki, or lucky-direction roll. This Kansai tradition has gotten quite popular around Japan recently, mainly due to convenience stores promoting them heavily. They are eaten on Setsubun no hi (first day of spring, February 3rd this year). Tradition holds that one must eat a whole roll, silently, while facing the lucky direction for that year.
A group of people doing the ehoumaki ritual, not quite solemnly.
Here's someone who looks a bit overwhelmed by her ehoumaki.
(More about ehoumaki: http://www.justhungry.com