The largest alkaline earth metal is barium (Ba). In the group of alkaline earth metals, which includes beryllium, magnesium, calcium, strontium, barium, and radium, size increases as you move down the group. This is due to the addition of electron shells as the atomic number increases.
Barium has the highest atomic number among these metals, which means it has more electron shells than the others. Consequently, it has a greater atomic radius, making it the largest alkaline earth metal. It is important to note that while radium is heavier, its atomic radius is slightly smaller due to the effects of electron shielding and relativistic effects in heavier elements.