Final answer:
The first recorded element to be discovered was phosphorus, isolated by Hennig Brand in 1669. In terms of atomic size, the order from smallest to largest for oxygen, sulfur, phosphorus, and potassium is oxygen < sulfur < phosphorus < potassium.
Step-by-step explanation:
The first recorded element to be discovered was phosphorus. The discovery of phosphorus predates the periodic table and modern chemistry. It was first isolated in 1669 by a German merchant and alchemist named Hennig Brand when he evaporated urine and heated the residue to a very high temperature.
Regarding the size of elements based on their position in the periodic table, the order of increasing size among oxygen, phosphorus, potassium, and sulfur would be: oxygen, sulfur, phosphorus, and potassium. The size increases down a group and decreases across a period. Since potassium is located below both sulfur and phosphorus in the same group, it is larger than both. Sulfur and phosphorus are both to the right of oxygen in the periodic table, making them larger than oxygen, but sulfur is closer to oxygen than phosphorus, making phosphorus larger than sulfur.