As of 2014, the atom on the periodic table of elements with the largest known radius is cesium, which has a radius of 298 picometers. For comparison, helium has a radius of 31 picometers. The radius has not been determined for all elements.
A picometer is one-trillionth of a meter, and is used to measure the distance of an atom's outermost valence electron shell from its atomic nucleus. Cesim has such a large radius because it is barely-bound together. The positive attraction of the atomic nucleus, which serves to keep the electron orbitals in place, falls off with distance. The electrons are not close enough to remain strongly bound, which is also why cesium is much more reactive than other elements.