Which is the largest atom with respect to potassium, oxygen, and silicon?
As we face the Table, atomic size decreases from left to right.......
As we face the Table, atomic size decreases from left to right ACROSS THE PERIOD.......and INCREASES DOWN A Group.
We have fourth Period germanium, Group 14, fourth Period potassium Group 1, third Period silicon, Group 14, and second Period oxygen, Group 16. Clearly, given the Periodicity, POTASSIUM is the LARGEST ATOM, in that it heads the Period.