* Atomic size trends: Atomic radius increases as you move down a group (column) on the periodic table and decreases as you move across a period (row) from left to right.
* Location on the periodic table: Selenium is in period 4 and oxygen is in period 2. Selenium is further down the periodic table, meaning its electrons are in higher energy levels and are further from the nucleus. This results in a larger atomic radius.
In summary: Selenium's atoms are larger than oxygen's atoms due to its lower position on the periodic table and the increased distance between its valence electrons and the nucleus.
