Hi.
Not done much mountain biking.
trying to fit new mtb

with road bikes, typically hand position shall be at shoulder width
does this rule of thumb go out the window with mountain bikes? i've heard it said that wider bars will be needed for off road control... but how wide?
And what if I'm going to be wierd and put drop bars on a mtb... shall I size the as if it were a road bike, or go wider than usual....

42cm is usual for me...

Advice?
Thanks