Atomic radius generally increases down a group and decreases across a period from left to right. Therefore, the element with the largest radius will be found in the bottom-left corner of the periodic table.
Hydrogen's atomic radius is exceptionally small because it has only one proton and one electron, and no electron shells beyond the first. It lacks the shielding effects present in larger atoms and has a very minimal electron cloud. Its position in the first period and its simple structure contribute to its tiny size compared to other elements.