My stance is about 65 cm, sometimes 70 in maneuvers and occasionally only 60, strapless. Being 5'9 or 175 cm.
Center to center of foot (or imaginary strap) that is.
Gunnar has around 60+13 = 73 stance measured center to center on his raceboards, and a lot less on small strapless.
The 13 cm is the difference: 16 cm between footstrap plugs and front straps angled so around 8 + 5 = 13 cm center to center of strap.
Another benifit of strapless is the ability to change ones stance at will.
Note that it seems to feel better to go fast being wider, even much wider, but for most general flying your normal surf stance should be comfortable.
High speed - wide stance is more stable.
Low speed - narrower/casual stance is more stable and comfortable.
It is that simple
Totally agree with the messages above.
When I race, my stance is as wide as the pad on my spotz board allows it to be ! It gives me better control but it's very tiring for the legs.