Sisteen gauge would be fine for someone operating up to perhaps 10 sound engines at a time. Each modern sound engine takes about 0.5 amps, often as little as half that and still be able to pull 10 cars up a 1.5% grade...ballparking.
If you were operating on a layout with others, and had many diesels consisted, say 12 or 16 around a layout with 300' of track, 16 gauge would probably still be okay, but getting marginal. 14 would be better, and once you begin to draw 10 amps or more consistently on your layout, 12 would be the better choice.
The way to calculate your amperage requirements is to have a fairly sound approximationg of the "stall current" requirements of your decoders. That is derived by clamping your engines with your hands and pressing down just enough to prevent them from moving. At the same time, you must have the tracks metered nearby, and turn the throttle up to a fairly high setting. Watch what the amp meter does (for DCC you need a special meter...the $14 multimeter from Wal Mart won't be accurate for the type of signal in DCC). You do this quickly, and then reduce the throttle within a second or two..you don't want to load the engine and decoder unnecessarily. That stall current reading, plus those of all your other engines, is your total requirement. I would add another 30% and call that your optimal requirement for a DCC system, and to figure out the load and wiring.
Generally, though, a regular non-sound DCC engine with a can motor will draw about 0.2-0.3 amps pulling a train typical of most layouts. Add those up, and you get your total current requirements. For sound engines, and they do vary, count on 0.4 as a safe bet, more if you know you'll have grades in excess of 2% and want to pull consists over 15 properly weighted cars.
