Basically, I would say medium.
The problem is when the foot is too far into the strap
The foot works as a lever on the board with the strap
as the fulcrum.
The foot/toe joints let you lock the foot in by riding position and foot flex, and release the board by leaning forward and extending the toes.
The line of max tension over the foot needs to be close enough to the toe joint line that toe extension will reduce pressure and let your foot pivot out.
Kind of simplified but that is how it works.
straps.GIF [ 42.36 KIB | Viewed 1100 times ]
So, I think tighter/smaller is better than too loose
It is really nice, if one foot comes out, to know the other foot can be kicked out too,
so the board does not become a giant torque wrench wrecking the other leg.
Some may want to lock their foot in more, for better freestyle or something
-the risk is breaking ankles and lower legs. Or use bindings and blow out a knee instead.
There are definitely times you want the feet secure on the board,
personally I'd rather blow a landing than an ankle.