1. The problem asks for the size of an interior angle of a regular polygon, specifically a regular hexagon which has 6 equal sides. 2. The formula for the measure of each interior angle of a regular polygon with $n$ sides is: $$\text{Interior angle} = \frac{(n-2) \times 180}{n}$$ 3. For a regular hexagon, substitute $n=6$: $$\text{Interior angle} = \frac{(6-2) \times 180}{6} = \frac{4 \times 180}{6}$$ 4. Simplify the expression: $$\frac{720}{6} = 120$$ 5. Therefore, each interior angle of a regular hexagon measures $120$ degrees.