Length
In general, a longe racket gives a longer swing, resulting in a more powerful shot. Longer rackets also naturally give a better reach. However, a longer racket is harder to control accurately, especially for smaller players.Grip Size
Choosing the correct grip size is very important to ensure the right feeling and allow for powerful shots. Too thick of a grip results in loss of power. A too thin grip size can on the other hand make it hard to hold the racket still enough, causing it to rotate in your hard, which is undesirable. The rule of thumb here is that when you are holding the racket comfortably, you should be able to touch the tip of your thumb to your middle finger knuckle.If you are unsure of which grip size to choose, picking one on the thinner side might be a good option, as you can always increase the grip size using grip tape later if needed. Generally male players use grip size 3 (4 3/8) while female players use grip 2 (4 1/4). But there are of course deviations from this as hand sizes vary.
Head Size
The head is the area containing the strings. There are standard-sized, mid-sized and over-sized rackets in this aspect. Standard-sized are nowadays rarely used as rackets have evolved towards larger head sizes. Mid-sized are most often used by more advanced players and offer a strung surface area of about 85-100 sq. inches. Over-sized rackets are very popular with beginners and more recreational players as their larger strung surfaces of about 100-140 sq. inches offers a considerably larger sweet spot.