What factors should I considering when selecting a tennis racquet?
There are a series of factors when considering to purchase a tennis racquets.  Primary factors include head size, length, weight, stiffness, and swingweight  

Head size - Power is closely correlated to head size - the larger the head size, the more power can be generated, all other factors being equal.  A larger head size also provides larger hitting area and 'sweetspot'.  Smaller head sizes tend to equate to more control and maneuverability. 

Length - The standard length is a 27 inches, but can range from 26.5 inches to 29 inches.  A longer racquets provides more reach and often results in more power.  Similar to head size, longer racquets may limit maneuverability and control. 

Weight - The heavier the racquet, the more power you will generate (increasing the 'swingweight') of the racquet.  The lighter the racquet, the more maneuverable and easier it will be for players to get the racket to the right swing position.  The balance of the racquet is also critical as part of weight.  A more head light racket (more weight in the handle) provides increased maneuverability, especially when using a heavier racquet. A more balanced or head heavy racket are often reserved for lighter racquets to maintain the mass behind the ball

Stiffness - Generally speaking, the stiffer the racket, the less energy is lost as the ball is redirected with your racquet and thus, creating more power.  More flexible racquets allow for more feel and control.  Flexible racquets provide more comfort as less shock and impact is transferred to the arm and elbow.

Swingweight - Swingweight measures how heavy a racquet feels when you swing it and the amount of power you can transfer to the ball.  Higher swingweight will also provide more stability in the racquet but will sacrifice maneuverability.