What is the maximum distance the power signal can travel in a cable?

If your design has the controller and relays at some distance from the motor and you will be running the risk of diminished voltage of the DC power as the distance increases. There are very good Voltage Drop Calculators on the web such as: http://www.calculator.net

Generally the thicker the wire size the greater amperage it can carry with less voltage loss. You will need to contact the motor manufacturer to see if there is a minimum voltage the motor can withstand and still operate.

Note: some 24VDC power supplies have adjustable voltage outputs that can be raised 2-3 volts to make up for line loss.