Special tips are 10cm long and while designed to make 40cm circular needles, using a 20 cm cable, they can be used with any length cable.
About Knitpro interchangeable cable and needle tip sizes:
Knitpro interchangeable tips are measured in two ways: needle thickness (eg 4mm, 4.5mm), and needle tip length (10cm, 13cm). Needle thickness is the same as any other knitting needles - thinner needles are used on thinner yarns, thicker needles are for thicker yarns (with some exceptions for particular effects or types of yarn).
The listed length of knitpro cables is approximately the total length from end to end with 10cm tips on, so with 13cm tips it will be a little bit longer than what is on the packet. Longer tips (13cm) can be more comfortable to use when knitting a larger piece such as the body of a jersey, while shorter tips (10cm) are useful for smaller circles such as sleeves and collars.