For X-country/trail riding I wouldn't think you need to go much wider than 680-720mm on the bar, do you have trees where you ride? Some trails I ride here in Georgia have some narrow openings between trees that my 680mm bar is close rubbing as I ride through.
Do you plan on getting riser bars or flat bars? That would affect what stem angle you would choose. For my XC riding, I tend to like riser bars with a 0deg, 100mm stem. I am about your size and that fits me comfortably.
I know everyone is thinking wide bars and short stem, however, that is coming from Enduro/Downhill riding where people like to move their weight a bit more off the front wheel and steering that favors high speed stability. For X-Country, I tend to like a more even front/rear weight distribution and steering that favors tighter lines.
Wide bars do offer a bit more leverage for long climbs, something favored by the single speed crowd.