The size of one degree is least on

Size of 1 degree depends on the length of 1 degree.

$\text{ Length of }1\text{ degree =}\frac{\text{Total length}}{\text{ Number of divisions}}$

For Celsius scale, $\text{ Length of }1\text{ degree =}\frac{\text{L}}{\text{ 100}}$

For Kelvin scale, $\text{ Length of }1\text{ degree =}\frac{\text{L}}{\text{ 100}}$

For Fahrenheit scale, $\text{ Length of }1\text{ degree =}\frac{\text{L}}{\text{ 180}}$

Hence the size of 1 degree is smallest on Fahrenheit scale.