Where the weight sits along the racket, measured in millimetres from the butt. Drag the balance point to feel the difference.
Even balance. A bit of everything: stable enough to attack, quick enough to defend. The safest choice if you are unsure. 30 rackets in stock sit within 2 mm of this.
Flex
How much the shaft bends as you swing. A bending shaft stores energy and gives it back, but only if your swing is quick enough to load it at the right moment.
Medium. Medium shafts suit most club players: some free power, good control, and forgiving when timing is off. Quick test: if your easy clear reaches the baseline, you can probably use a firmer shaft.
String tension
Tighter strings give more control and a crisper feel but a smaller sweet spot. Looser strings are more forgiving and kinder on the arm.
24 lbs. Club range: a good balance of forgiveness and control. Plastic shuttles and cold halls suit a pound or two lower.
Grip size
Hold the racket in a relaxed forehand grip. There should be about a finger's width between your fingertips and the base of your thumb. No gap means the handle is too thick; a big gap means it is too thin.
Yonex G3
About 89 mm around
Yonex G4
About 86 mm around · common in the UK
Yonex G5
About 83 mm around · common in the UK · common in Asia
Yonex G6
About 78 mm around · common in Asia
Handles can be built up with overgrips but never made thinner, so between two sizes, choose the smaller. Victor and Li-Ning G sizes are close to Yonex's; old Li-Ning S sizes run the other way.