Step 1: Split the factors into groups.
Things that change runoff fall into two groups: climatic factors (about the rain and weather) and physiographic factors (about the land itself).
Step 2: Spot the climatic ones.
Duration of rainfall, rainfall intensity and type of precipitation all describe the rain. So these are climatic, not physiographic.
Step 3: Recall what physiographic means.
Physiographic factors describe the physical shape of the catchment, such as its size, shape, slope and soil.
Step 4: Match the option.
Size of the watershed is a property of the land, so it is a physiographic factor.
Step 5: Confirm the logic.
A larger watershed collects water from a bigger area and changes the timing and amount of runoff, which fits a land based factor.
Step 6: Conclusion.
The physiographic factor among the options is the size of the watershed.
\[ \boxed{\text{Size of watershed}} \]