Like the other two members have mentioned- it's 100% preference. With that being said, there is still a range in which you could capture all the stance widths.
Generally speaking, wider stances offer you better weight distribution and a lower center of gravity which lend themselves to better balance overall. This is desirable for balancing on rails and boxes.
The more narrow a stance, the easier it is to carve at higher speeds or spin in the air.
To some degree, stance width and board length tend to be inversely proportional.
As a good guideline, I would suggest starting with your feet just a bit wider than shoulder length. I'm 6' and that puts me at a stance of about 21". I have ridden closer to 22" or 22.5" recently because it feels better for me.
Bring a screwdriver or whatever binding tool you have and a tape measure. You can bump the stance out or draw it in every couple runs. You will probably be fine-tuning this measurement for a long time.