On a side note I got one for my car it's flat 6 foot long and said it's rated 2 amps but it never fast charges my phone even though my car has a fast charger.
We cannot answer that for you.Test it out! - If it works as intended and the voltage delivered at the Arduino isn't too low, it's fine.It isn't going to break on it's own just because it's skinny. It will break if you flex it a lot.// Per.
Voltage is not dependent on the cable. Voltage is dependent on the wall-wart, except for voltage drop, which in this case will be negligible given the cable is only 30cm long.
New USB Short 2.0 A Male to Mini 5 Pin B Data Charging Cable Cord Adapter
Description100% Brand New and High QualityColor: BlackLength:50cmNew generic USB 2.0 Cable, Type A to Mini 5-Pin Type BQuantity: 1Transfer images and data between your camera / cell phone to computer by USB portConnectors: USB to Mini USB 5-PinUSB 2.0 specification, Hi-Speed transfer rates up to 480MbpsPC and Mac compatiblePackage Includede1 x High-speed USB A Male to B Mini 5 Pin Sync Cable