Power
The main factors that govern the power of a racquet are head size and frame flexibility. Lower string tension also seems to increase power, but in fact, it makes the ball fly farther not because of more power but because looser strings release the ball later in the swing, when the racquet has tilted upward slightly more. An inexpensive racquet will come pre-strung at the middle of its tension range, and you should probably choose mid-range for your first custom stringing, too. That leaves head size and flexibility to consider as the real determinants of power.
A larger head gives you more power and a larger sweet spot, but less control. Most racquets come in one of three basic sizes: A midsize has a hitting area of 85-95 square inches, mid-plus 95-105 square inches, and oversize greater than 105 square inches. If your athletic ability is above average, choose mid-plus; otherwise, choose an oversize up to 115 square inches. Anything larger will be so powerful, it will discourage you from taking a real swing at the ball, because when you do, you'll too often hit long. A few pros use oversize racquets, but they're most commonly designed for beginners. Midsize and mid-plus are usually preferred by intermediate and advanced players.
For a beginner, flexibility won't make as big a difference as head size. A more flexible racquet gives you somewhat less power and slightly less control, but until you start hitting hard and trying to place the ball within a few feet of a target, you probably won't notice. All aluminum racquets are somewhat flexible, but graphite racquets range from flexible to extremely stiff. Generally, the thicker the profile, the stiffer the frame, but the frame materials and construction matter, too. If you're going to spend the money for graphite, a moderately stiff to stiff frame is probably your best bet.
Length
The standard length for an adult racquet is 27 inches. Anything shorter is meant for a junior. Racquets longer than 27 inches emerged several years ago, intended to give players more reach and leverage. The merits of extra-long racquets are hotly debated, with greater serving power argued as the main advantage and reduced maneuverability the main criticism. If you're not very tall, an extra inch of racquet can enhance your serve, and it shouldn't feel unwieldy, but don't make length your main consideration. Between 27 and 28 inches, the difference won't be crucial. Any length above 28 inches is probably unwise for a first racquet.
