Are Kerry Washington's feet considered very big since her shoe size is 8 and she is only 5'4.5?

Yes her feet are considered large for her size of 5'4.5". Petite woman's shoe size ranges from 6.5 and lower.

However it depends on country. In the Asia-Pacific region the average shoe size is 6.5 and lower. But in more Western regions like South Asia (India, Iran . . .), Western Europe, North America, South America, and Africa the average shoe size is 8 because women tend to be taller and bigger boned. Kerry is considered petite but her genes most likely contribute to a larger shoe size or bigger foot for a woman.

This is a generalization and can have variation.