I am 5'9", 165 lbs, and wear a size 12 boot. Every board I have ever owned was a standard width and my boots fit fine on it with a +20,-10 stance. In your situation I would get a standard width board. Wide boards are more difficult to initiate turns especially if you only weight 140 and you will probably find it even harder to go from edge to edge. Generally a wide board is required if your toes are over hanging the edge of the board and you find yourself scraping the toe of your boot along the snow when doing carving toe-side turns. Since you said you were at a beginner/ intermediate level, most of your turns are probably skidded turns or scarving which a slight toe overhang won't really effect you too much.