When trying to determine which atom has the biggest atomic radius, there are two things you have to consider: 

1) As you move across the periodic table from left to right, the atoms get smaller. This is due to the addition of a positively charged proton to each element. In turn, that proton attracts the electrons with greater significance than the previous element and therefore making it smaller. 

2) As you move down the periodic table from top to bottom, the atoms get bigger. From one period to the next, the valence electrons occupy a shell that's further away from the nucleus. The further the valence electrons are, the harder it is for the nucleus to pull them closer. This is called electron shielding.

For more information, visit our atoms page!