One of the most common problems when wiring in a motor controller is that the user inadvertently strips either too short or too long a length of wire.
Too short and it can become hard to get a good connection in a screw connector (and especially hard if a soldered connection is required). Too long and you run the risk of the cables shorting either on each other, on the driver itself or on something else in the application. Depending on the motor controller that you are using, a short like this could completely blow the controller.
Typically, 5-10mm is a good length but this will vary depending on the type of connector that you are using.