How can you tell which element is the largest by looking at their electron configuration?

Answer 1

You need to know more than just the electron configuration. There is contraction from left to right on the periodic table, plus additional periodic trends that must be considered.

Even more importantly, you need to define whether you are looking for the largest element IN ITS NATURAL STATE (i.e. Cl2, Hg metal, etc) or AS THE ION. Atomic and ionic radii differ.

More clarity is needed here.

Answer 2

I'm assuming your talking atoms here not ions. The largest atomic radius would be located in the lower left hand corner of the periodic table. This is Francium.