You just have to follow the USB 2.0 standard (however recommend getting one that has good shieling), anyway I believe the maximum length is 5 meters (or about 16 feet), but you can actually make an even longer USB extension cable if use USB to Ethernet/RJ-45 extender adapter/converter kit (but again recommend get a shielded cable for it). That and more discussed here → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage