experiment...

generally, I would recommend that your feet be pointing in the same direction, parallel to each other through the entire stroke.
unless you are extremely duck or pigeon footed.

if it's too narrow, your toes will point at each other
if it's too wide, your toes will point away from each other.


now, having all said that, there are MTBs that have some super wide cranks due to clearance issues, and I haven't really heard of anyone complaining about how wide these cranks are.