Are you sure 9/16 is too small? Crank bolts are usually 14mm, not 15.
The classic Campy bolts were 15mm.
Most Shimano bolts I've found were 14mm.
One thing to keep in mind is that a 15mm socket is a tight squeeze to get into the crank arm.
If you're buying a socket, then make sure it fits. I ground a socket down in the past, but I've also found thinwall sockets that will work. Take a crank arm with you to the hardware shop
