As the size of the head is directly proportional to the speed of the ball on impact with the strings, you need to know what is best for your game. Basically, the larger the head, the bigger the sweet spot, and the faster the ball will spring off the strings.
Racket head size refers to the total surface area of the head and is usually expressed in square inches.
If you are working up to the intermediate level, 102-105 square inches will likely be the best size. Players seeking more power are advised to go for an oversize racquet of 105-115 square inches with an open string pattern, but this may come at the sacrifice of control.
The larger sweet spot makes off-center shots less problematic, allowing for more consistent power shots.