A grip that is too big or too small could lead to wrist and/or arm injuries. (See Tennis Elbow)
It is best to buy a grip too small for you if you are unsure. The grip can then be enlarged
by fitting a thicker base grip or by adding overgrips. A good way to measure your grip size is shown in more detail on the Grip Sizing page.
The maximum length of a racket according to the Rules of Tennis is 29.0 inches. Most tournament players play with a standard length racket (27 inches), but the extra reach and power (particularly on the serve and smash) offered by longer-framed rackets (between 27.5 and 28.5 inches) are desirable features for some players. You will, however, loose some manoeuvrability on the longer rackets.
Rackets that are pre-strung when purchased often have cheaper, inferior strings. Restringing the racket before using it will enhance the playability of the racket. A gauge number is used for string thickness (usually accompanied by the string's thickness in mm). The majority of strings measure between 15-17 gauge. Thinner strings (high gauge numbers) are more responsive but have a shorter life-span; thicker strings have the opposite characteristics. You can increase or decrease the power of your racket, to some degree, by altering the tension when restringing your racket. String tension is usually measured in lbs or kgs per square inch. (See the page on String Tensions for more information.) A low tension (45 lbs per square inch) would increase the power of your racket while lessening the control. A high tension (62 lbs per square inch) would decrease power while increasing control. Find a string and string tension that suits your playing requirements. Strings come in a huge variety of colours and constructions and which are described in more detail on the Strings Construction page.
The heavier the racket the more potential it has to generate power, but the harder it is to manoeuvre. Heavier rackets tend to feel more solid when hitting a ball but it is harder to generate racket speed.
The larger the head size, the larger the sweetspot, and the greater the power that can be generated. However a smaller head size gives you more control. A standard mid-sized head is typically 95 square inches. Advanced players with big swings in their groundstrokes tend to favour smaller headsizes as they are not looking for help in generating power. Again, the maximum size of the head of the racket is stated in the Rules of Tennis.
A wide frame provides more power but less control. A standard width is typically 22mm. Some rackets have a straight beam frame (the width is constant throughout the length of the racket) while others have a tapered beam (the width varies along the length of the racket). A thin framed racket (19mm) will enhance feel for the ball as there is a better feedback from the racket to the player.
A stiffer frame generates more power, but again, the disadvantage is reduced control. A stiff frame transmits more shock to the arm after impact than a more flexible frame would. Most rackets have in built anti-vibration features to reduce this, whether in the racket grip or in the frame. If you still feel too much vibration down the racket then you can buy vibration dampers (or 'dumbies') that can be placed on your racket strings. These vibration dampers must be placed outside of the main string pattern in accordance with the rules and come in many varieties. Stiffness is measured by the 'Flex' rating - the higher the number, the stiffer the racket.
For a resumé of Basic Facts see the Frame and String Facts page.
Swingweight might be the most important factor in the feel and performance of a racquet. It takes into account the weight, the distribution of the weight, and the torque necessary to accelerate a racquet of given weight distribution. It is an indicator of feel, maneuverability and power. A higher swingweight has more power at the same swing speed. However it is also more difficult to accelerate the racquet up to that speed. If you cannot maintain racquet speed at the increased swingweight, you will actually lose power. It all depends on what you are looking for. If you are a big serve and volley player, you might want a lower swingweight racquet so you can generate lots of head speed for the serve and have maximum maneuverability at the net. If you are a baseliner and can generate plenty of racquet speed at any swingweight, then a higher swingweight might be for you.
Comparison of Frames
A comprehensive and interactive table which allows for comparison of many frames is available here.