Question

which element has largest atomic size among berylium carbon and oxygen

Answer 1

oxygen has largest atomic size because it have high atomic number

Answer 2

Beryllium has the largest atomic radius among the atoms of Carbon and Oxygen because all these three elements lie in the same period in the Modern Periodic Table and beryllium comes to the left of the period horizontally and also it has only 4 electrons while carbon has 6 electrons and Oxygen has 8 electrons
So as the nuclear charge increases the pull of the nucleus towards the atomic shells increases.
That's why as the nuclear charge increases the. atomic size decreases.
And hence beryllium has the largest atomic radius and atomic size among carbon and Oxygen too.