Choosing the right width is perhaps the trickiest factor.  For me personally, I use extra wide because I carve deep with using low stance angles and do not want to 'boot-out', so going extra wide is the best solution for me.  Here's some factors with regards to width:

• Boot size.  Large boot sizes may need wide to not 'boot-out' during carving.
• How well you carve.  If you aren't actually doing carved turns, a wider board really isn't going to help unless you have a lot of toe or heel overhang.  If you really are carving and leaving a single line in the snow, then a wider board will help if you are having issues with your boots or bindings hitting into the snow.
• Snow conditions.  In Colorado we generally have softer snow that leaves up to a couple inch 'trench' during carved turns.  With making such a track, the 'boot-out' potential increases, so wider is my preference.  On very firm snow, such as the Mid-West and East Coast generally have, the 'trench' into the snow won't be as deep and generally won't need as wide of a board to avoid 'boot-out'.
• How you want to ride.  If progressing towards deeper and deeper carved turns is your goal, then wider and wider boards has worked perfectly for me.  If you're not planning on carving deeper and not having issues with hitting your boots or bindings on turns, then a wider board probably isn't justified.