Rafflesia Arnoldii is often over 3 feet wide and its petals are almost
an inch thick. You wouldn't want it in your garden, however, because
it smells like a rotting animal.

Has anyone grown it in a garden? Not as far as I know (it is an
obligate parasite of certain specific plants).

The Titan Arum, on the other hand, another large carrion flower, is
grown in captivity, with increasing success in recent years:
http://en.wikipedia.org/wiki/Titan_arum