as you have not given us an idea of the rating of the steppers , take this as a guide , you need to keep the wire to a size able to support the Current ( AMP) of the stepper , and also take into account the length of wire also , the longer the wire the greater the loss , so keep the diameter of the wire as large as practical , i.e that will fit the connectors etc , although for most use the majority of steppers are about 4-6A Current , so the ideal size would be say cable marked for a Current capability of 6 or 10A , these are usualy expressed as in the catalogue as Equipment wire for single cores and in sizes expressed as number of wires / dia of wire
just to make it more confusing , but as a guide if the cable is not marked in it's current capability here's a guide , it's better to go larger in size than to go smaller

Equipment wire ( single core )
24/0.2 = 6A
7 /0.2 = 12A
50 / 0.25 = 25A

for multicore cables
16 /0.2 = 12A

look at the wires coming out of the stepper for a visual guide , and look for wires around the same or larger in diameter