I'd rather the bars be narrower than wider. But I've only ridden road. Don't know what might be different if it was on a cross bike on fairly rough conditions.

If I were to size based on shoulder width of 17.25 then I'd be on 44 cm bars. But I prefer my 38 cm bars on my Raleigh though I've been riding 42 cm bars on my Paramount the most lately. Not a big difference but I can't see wanting wider. I don't like my hand being that far out when one handed on the brake/shifter and trying to get a drink.

If you haven't already, be sure you discuss why with the fitter. They might know something about you that we don't.