dont use the adapter, cut the plug off and wire it directly to the 12v cord in the dash.
and you can use any length you want, but over 24 you're gonna get power spikes and bad data. just make your own power cable, its very simple, just cut and solder. dont put the dvd drive in the armrest though, put it in the trunk or near the backseat. how many dvd's do you plan on watching on your way to work? the whole point is so you dont have to swap discs.